導數專題 函式零點個數
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基礎知識
函式的零點、方程的實數根
函式的零點、方程的實數根與兩函式的交點可視為同一問題,
函式\(h(x)=f(x)-g(x)\)的零點個數
\(⇔\)方程\(f(x)=g(x)\)實數根個數
\(⇔\)函式\(y=f(x)\)與函式\(y=g(x)\)交點個數.
函式零點存在定理
如果函式\(y=f(x)\)在\([a ,b]\)上的圖象是連續不斷的,且\(f(a)f(b)<0\),那麼函式\(y=f(x)\)在\((a ,b)\)至少有一個零點\(c\),即存在\(c∈(a ,b)\),使得\(f(c)=0\),這個\(c\)也就是方程\(f(x)=0\)的解.
求函式零點個數的方法
對於複雜的函式(特別是含參函式)需要利用導數的方法求解
(1) 直接法
直接法,即直接對所求函式進行分析,
(i)對於不含參函式,比如\(f(x)=x^3-2x^2-x\),\(f(x)=e^x-2x^2+x\)等,
求零點個數的思路是:求導→求單調性→求極值最值→結合函式圖象分析零點個數;
(ii)對含參函式,進行分類討論就行.
(2) 分離引數法
對於求含參函式的零點個數,採取分離引數法可把問題轉化為求不含參函式問題;
比如求函式\(f(x)=e^x-ax\)的零點個數,
採取分離引數法相當於求函式\(y=\dfrac{e^x}{x}\)與函式\(y=a\)的交點個數.
(3) 切線法
切線法,即利用導數的幾何意義求兩函式相切的“臨界值”,再結合圖象判斷交點個數;
若所求函式能“分離”出一次函式或“分離”兩函式有明顯“凹凸性”,可考慮切線法;
比如求函式\(f(x)=e^x-ax\)的零點個數,可轉化為\(y=e^x\)與\(y=ax\)的交點個數,
此時先確定當\(a\)為何值時兩函式相切,再結合圖象分析交點個數.
比如求函式\(f(x)=e^x+1-a\left(2+\dfrac{x}{e^x}\right)\)
注意事項
(1) 求函式零點個數時,時常要結合函式圖象,所以儘量圖象準確,不要想當然;
【例】求函式\(f(x)=xe^x\)的零點個數.
(注 函式零點明顯僅有\(0\),以下討論僅為表達畫圖的重要性)
\(\because f'(x)=(x+1)e^x\),
\(\therefore\)函式\(f(x)\)在\((-∞,-1)\)遞減,在\((-1,+∞)\)遞增,最小值\(f(-1)=-\dfrac{1}{e}<0\),
此時就以為函式圖象是左圖,那就得到“有兩個零點”的錯誤結論,
嚴謹的表達是,\(\because x⟶-∞\),\(f(x)⟶0\),\(\therefore f(x)\)在\((-∞,-1)\)不存在零點,
\(\because f(1)=e>0\), \(\therefore f(-1)f(1)<0\),
由函式零點存在定理可得\(f(x)\)在\((-1,+∞)\)存在\(1\)個零點,
故函式\(f(x)=xe^x\)實際的圖象如右圖,僅有一個零點.
(2) 要避免上訴“想當然”的情況需要在零點旁邊找兩個函式值異號的點,方法有二種,
(i)利用極限的思路,比如判斷\(x→x_0\)時,\(y→y_0\);\(x→-∞\)時,\(y→+∞\)之類的;
此時會涉及到函式增長快慢問題,但若涉及大學的內容(比如洛必達法則),則解答不會得滿分;
(ii) 找到“實實在在”的點;
(3) 對某些含參函式,分離變數的方法行不通,比如因為需要洛必達法則、求導困難等,需要用直接法進行分類討論.
極限問題
(1) 函式的增長速度
一般地,如果一個函式在某一範圍內導數的絕對值\(|f'(x)|\)較大,那麼函式在這個範圍內變化得較快,這時函式的圖象就比較“陡峭”(向上或向下);反之,函式在這個範圍內變化得較慢,函式的圖象就比較“平緩”.
【例】指數函式\(y=a^x (a>1)\)在\((0,+∞)\)的增長速度是“爆發式”的,
冪函式\(y=x^n (n>0)\)在\((0,+∞)\)的增長速度較快,指數\(n\)越大,增長速度越快.
一次函式\(y=kx+b(k>0)\)的增長的速度不變,\(k\)越大,其增長得越快.
對數函式\(y=\log _ax(a>1)\)在\((1,+∞)\)的增長速度很慢.
(2) 極限
對於某函式\(y=f(x)\),當\(x→a\)時,\(y\)趨向什麼”屬於極限問題,本質是求\(\lim _{x \rightarrow a} f(x)\).
以下舉幾個例子,大家細品下,
① \(f(x)=e^x\),當\(x→0\)時,\(y→e^0=1\);即函式\(y=e^x\)能取到\(0\),直接代入便可;
② \(f(x)=\dfrac{1}{x}\),當\(x→+∞\)時,\(y→0\);可想象下\(x\)取一很大的數\(10000\),對應函式值\(y=\dfrac{1}{10000}\)很小,接近\(0\);
③ \(f(x)=\dfrac{\ln x}{x^2}\) ,當\(x→+∞\)時,分子\(\ln x→+∞\),分母\(x^2→+∞\),那\(\dfrac{\ln x}{x^2}\)趨向什麼呢?
因為函式\(y=\ln x\)較函式\(y=x^2\)在\((0,+∞)\)增長得慢很多,所以當\(x→+∞\)時, \(\dfrac{\ln x}{x^2} \rightarrow 0\);
這需要了解函式間在某區間的增長速度的比較;
④ \(f(x)=\dfrac{\sin x}{x}\),當\(x→0\)時,分子\(\sin x→0\),分母\(x→0\),那\(\dfrac{\sin x}{x}\)趨向什麼呢?
此時\(y=x\)與\(y=\sin x\)在\(x=0\)附近的增速在高中無法確定,
其實\(x→0\)時, \(\dfrac{\sin x}{x} \rightarrow 1\)的,為什麼呢?(用洛必達法則可求,但用高中導數的定義也可求)
其實\(\lim _{x \rightarrow 0} \dfrac{\sin x}{x}=\lim _{x \rightarrow 0} \dfrac{\sin x-0}{x-0}=\lim _{x \rightarrow 0} \dfrac{g(x)-f(0)}{x-0}=g^{\prime}(0)=1\),(\(g(x)=\sin x\),\(g'(x)=\cos x\))
如下圖
基本方法
【題型1】 求不含參函式的零點個數
【典題1】判斷\(g(x)=2\ln x-\dfrac{1}{2} x^2+x\)的零點個數.
解析 \(g(x)=2\ln x-\dfrac{1}{2} x^2+x\)的定義域是\((0,+∞)\),
\(g^{\prime}(x)=\dfrac{2}{x}-x+1=\dfrac{(-x+2)(x+1)}{x}\),
由\(g'(x)=0\)得\(x=2\),
所以當\(x∈(0,2)\)時,\(g'(x)>0\),\(g(x)\)單調遞增;
當\(x∈(2,+∞)\)時,\(g'(x)<0\),\(g(x)\)單調遞減,
所以\(x=2\)是\(g(x)\)唯一的極值點且為極大值點,
故\(g_{\max }(x)=g(2)=2 \ln 2>0\),
又\(g\left(\dfrac{1}{e}\right)=-2-\dfrac{1}{2 e^2}-\dfrac{1}{e}<0\),
\(g\left(e^2\right)=4-\dfrac{1}{2} e^4+e^2=-\dfrac{1}{2}\left(e^2-1\right)^2+\dfrac{9}{2}<0\),
由零點的存在性定理知,\(g(x)\)在\(\left(\dfrac{1}{e}, 2\right)\)和\((2,e^2 )\)上分別有一個零點,
故\(g(x)\)有\(2\)個零點.
點撥 解題時多結合導函式“穿線圖”與原函式“趨勢圖”,關於本題的取點,在\((0,2)\)上取\(x=\dfrac{1}{e}\),在\((2,+∞)\)上取\(x=e^2\),取點一要看取點範圍,二看函式特徵,而函式中含\(\ln x\)多取\(x=1\)、\(e\)、\(\dfrac{1}{e}\) 、\(e^2\)之類的,若失敗了多嘗試,比如本題中取\(g(e)=2-\dfrac{e^2}{2}+e>0\)失敗就再取大些的\(x=e^2\)等.若本題是非解答題,不要取點,直接由\(x→-∞\),\(y→-∞\)和\(x→+∞\),\(y→-∞\)可得.
【鞏固練習】
1.在函式\(f(x)=ax^3+bx(a≠0)\)影象在點\((1,f(1))\)處的切線與直線\(6x+y+7=0\)平行,導函式\(f'(x)\)的最小值為\(-12\).
(1)求\(a\)、\(b\)的值;
(2)判斷方程\(f(x)=0\)解的個數.
2.證明函式\(f(x)=\ln (x+1)-\sin x\)在\(\left(\dfrac{\pi}{2},+\infty\right)\)上有且僅有一個零點.
參考答案
-
答案 (1)\(a=2\),\(b=-12\);(2)\(3\).
解析 (1)\(a=2\),\(b=-12\)(過程略).
(2)由(1)知\(f(x)=2x^3-12x\),
\(\therefore f^{\prime}(x)=6 x^2-12=6(x+\sqrt{2})(x-\sqrt{2})\),列表如下:
\(x\) | \((-∞,-\sqrt{2})\) | \(-\sqrt{2}\) | \((-\sqrt{2},\sqrt{2})\) | \(\sqrt{2}\) | \((\sqrt{2},+∞)\) |
---|---|---|---|---|---|
\(f'(x)\) | \(+\) | \(0\) | \(-\) | \(0\) | \(+\) |
\(f(x)\) | ↗ | 極大值 | ↘ | 極小值 | ↗ |
\(\therefore f(x)\)的極大值是\(f(-\sqrt{2})=8\sqrt{2}\),極小值是\(f(\sqrt{2})=-8\sqrt{2}\),
且當\(f(-3)=-18<0\),\(f(3)=18>0\),
\(\therefore f(-3)\cdot f(-\sqrt{2})<0\),\(f(3)\cdot f(\sqrt{2})>0\),
\(\therefore f(x)\)在\((-∞,-\sqrt{2})\)、\((-\sqrt{2},\sqrt{2})\)、\((\sqrt{2},∞)\)上各有\(1\)個零點,
故函式\(f(x)\)有三個零點,即方程\(f(x)=0\)解的個數是\(3\).
- 證明 \(f^{\prime}(x)=\dfrac{1}{x+1}-\cos x\),
當\(x \in\left(\dfrac{\pi}{2}, \pi\right)\)時,\(-\cos x>0\), \(\dfrac{1}{x+1}>0\),
\(\therefore f^{\prime}(x)=\dfrac{1}{x+1}-\cos x>0\),\(\therefore f(x)\)單調遞增,
而\(f\left(\dfrac{\pi}{2}\right)=\ln \left(\dfrac{\pi}{2}+1\right)-1<0\),\(f(π)=\ln (π+1)>0\),
所以\(f(x)\)在\(\left(\dfrac{\pi}{2}, \pi\right)\)上有且僅有一個零點\(x_0\);
當\(x∈[π,+∞)\)時,\(f(x)=\ln (x+1)-\sin x>\ln (π+1)-1>0\),
所以以\(f(x)\)在\([π,+∞)\)上無零點;
綜上所述,\(f(x)\)有且僅有一個零點.
【題型2】 分離引數法
【典題1】 若\(f(x)=ae^2x+(a-2)e^x-x\)有兩個零點,求\(a\)的取值範圍.
解析 依題意, \(f(x)=0 \Leftrightarrow a\left(e^{2 x}+e^x\right)=2 e^x+x \Leftrightarrow a=\dfrac{2 e^x+x}{e^{2 x}+e^x}\),
令\(g(x)=\dfrac{2 e^x+x}{e^{2 x}+e^x}\),
則\(g^{\prime}(x)=\dfrac{\left(2 e^x+1\right)\left(e^{2 x}+e^x\right)-\left(2 e^x+x\right)\left(2 e^{2 x}+e^x\right)}{\left(e^{2 x}+e^x\right)^2}=\dfrac{\left(2 e^x+1\right)\left(1-e^x-x\right)}{e^x\left(e^x+1\right)^2}\),
令\(h(x)=1-e^x-x\),
顯然函式\(h(x)\)是\(R\)上的減函式,而\(h(0)=0\),
當\(x<0\)時,\(h(x)>0\),\(g'(x)>0\),當\(x>0\)時,\(h(x)<0\),\(g'(x)<0\),
因此,函式\(g(x)\)在\((-∞,0)\)上單調遞增,在\((0,+∞)\)上單調遞減,
當\(x=0\)時,\(g(x)_{max}=g(0)=1\),
而\(g(-1)=\dfrac{2-e}{e^{-1}+1}<0\),
又當\(x>0\)時,\(g(x)>0\)恆成立,
函式\(f(x)\)有兩個零點,等價於直線\(y=a\)與函式\(y=g(x)\)的圖象有兩個公共點,
在同一座標系內作出直線\(y=a\)與函式\(y=g(x)\)的圖象,如圖,
由圖象知,當且僅當\(0<a<1\)時,直線\(y=a\)與函式\(y=g(x)\)的圖象有兩個公共點,
所以\(a\)的取值範圍是\((0,1)\).
點撥 利用分離引數法,能把含參函式問題轉化為不含參函式問題.
【鞏固練習】
1.已知函式\(f(x)=(x-a)\ln x(a∈R)\),若函式\(f(x)\)存在三個單調區間,則實數\(a\)的取值範圍是\(\underline{\quad \quad}\) .
2.討論函式\(g(x)=\dfrac{1}{x}-\dfrac{m}{x^2}-\dfrac{x}{3}(x>0)\)零點的個數.
3.已知函式\(f(x)=\dfrac{x^2+2 x+4}{x+2}\).
(1)求函式\(f(x)\)在區間\([-1,1]\)上的最值;
(2)若關於\(x\)的方程\((x+2)f(x)-ax=0\)在區間\((0,3)\)內有兩個不等實根,求實數\(a\)的取值範圍.
參考答案
-
答案 \(\left(-\dfrac{1}{e^2}, 0\right)\)
解析 \(f^{\prime}(x)=\ln x+\dfrac{1}{x}(x-a)=\ln x+1-\dfrac{a}{x}\),
函式\(f(x)=(x-a)\ln x(a∈R)\),若函式\(f(x)\)存在三個單調區間,
即\(f'(x)=0\)有兩個不等實根,即\(a=x(\ln x+1)\)有兩個不等實根,
轉化為\(y=a\)與\(y=x(\ln x+1)\)的影象有兩個不同的交點,
\(y'=\ln x+2\),令\(\ln x+2=0\),即\(x=\dfrac{1}{e^2}\) ,
即\(y=x(\ln x+1)\)在\(\left(0, \dfrac{1}{e^2}\right)\)上單調遞減,在\(\left(\dfrac{1}{e^2},+\infty\right)\)上單調遞增 ,
\(y_{min}=-\dfrac{1}{e^2}\),
當\(x\in \left(0, \dfrac{1}{e^2}\right)\)時,\(y<0\),
所以\(a\)的範圍為\(\left(-\dfrac{1}{e^2}, 0\right)\). -
答案 當\(m>\dfrac{2}{3}\)時,函式\(g(x)\)無零點;
當\(m=\dfrac{2}{3}\)或\(m≤0\)時,函式\(g(x)\)有且僅有一個零點;
當\(0<m<\dfrac{2}{3}\)時,函式\(g(x)\)有兩個零點.
解析 令\(g(x)=0\),得\(m=-\dfrac{1}{3} x^3+x(x>0)\),
設\(h(x)=-\dfrac{1}{3} x^3+x(x>0)\),
所以\(h'(x)=-x^2+1=-(x-1)(x+1)\),
當\(x\in (0,1)\)時,\(h'(x)>0\),此時\(h(x)\)在\((0,1)\)上為增函式;
當\(x\in (1,+∞)\)時,\(h'(x)<0\),此時\(h(x)\)在\((1,+∞)\)上為減函式,
所以當\(x=1\)時,\(h(x)\)取極大值\(h(1)=-1+\dfrac{1}{3}=\dfrac{2}{3}\),
且\(x→0\)時\(h(x)→0\),\(x→+∞\)時\(h(x)→-∞\),
故當\(m>\dfrac{2}{3}\)時,函式\(y=m\)和函式\(y=h(x)\)無交點;
當\(m=\dfrac{2}{3}\)時,函式\(y=m\)和函式\(y=h(x)\)有且僅有一個交點;
當\(0<m<\dfrac{2}{3}\)時,函式\(y=m\)和函式\(y=h(x)\)有兩個交點;
當\(m≤0\)時,函式\(y=m\)和函式\(y=h(x)\)有且僅有一個交點.
綜上所述,當\(m>\dfrac{2}{3}\)時,函式\(g(x)\)無零點;
當\(m=\dfrac{2}{3}\)或\(m≤0\)時,函式\(g(x)\)有且僅有一個零點,
當\(0<m<\dfrac{2}{3}\)時,函式\(g(x)\)有兩個零點. -
答案 (1)最大值為\(3\),最小值為\(2\);(2) \(\left(6, \dfrac{19}{3}\right)\).
解析 (1)最大值為\(3\),最小值為\(2\)(過程略).
(2)因為關於於\(x\)的方程\((x+2)f(x)-ax=0\)在區間\((0,3)\)內有兩個不等實根,
則\((x+2) \cdot \dfrac{x^2+2 x+4}{x+2}-a x=0\)在區間\((0,3)\)內有兩個不等實根,
所以\(x^2+2x+4-ax=0\)在區間\((0,3\))內有兩個不等實根,
所以\(a=\dfrac{x^2+2 x+4}{x}\)在區間\((0,3)\)內有兩個不等實根,
令\(g(x)=\dfrac{x^2+2 x+4}{x}\),\(x\in (0,3)\),
\(g(x)=x+2+\dfrac{4}{x} \geqslant 2 \sqrt{x \cdot \dfrac{4}{x}}+2=6\) (當且僅當\(x=2\)時,取等號),
\(x→0\)時,\(g(x)→+∞\); \(g(3)=\dfrac{3^2+2 \times 3+4}{3}=\dfrac{19}{3}\),
所以\(a\)的取值範圍為 \(\left(6, \dfrac{19}{3}\right)\).
【題型3】直接法
【典題1】 討論函式\(f(x)=x\ln x-\dfrac{1}{2} x^2+(a-1)x(a\in R)\)的極值點的個數.
解析 \(f(x)\)的定義域是\((0 ,+∞)\),\(f'(x)=\ln x-x+a\),
令\(g(x)=\ln x-x+a\),則\(f'(x)=\ln x-x+a\),(建構函式,二次求導)
當\(x\in (0 ,1)\)時,\(g^{\prime}(x)>0\),\(g(x)\)單調遞增,即\(f'(x)\)單調遞增;
當\(x\in (1 ,+∞)\)時,\(g'(x)<0\), \(g(x)\)單調遞減,即\(f'(x)\)單調遞減;
所以當\(x=1\)時,\(f'(x)\)有極大值\(f'(1)=a-1\),也是最大值,
(確定\(f'(x)\)的最大值\(a-1\),想下函式圖象\(a-1\)與\(0\)的大小比較決定導函式\(y=f'(x)\)是否存在零點)
① 當\(a-1≤0\),即\(a≤1\)時,
所以\(f(x)\)在\((0 ,+∞)\)上單調遞減,此時\(f(x)\)無極值,
② 當\(a>1\)時,\(f'(1)=a-1>0\),
\(f^{\prime}\left[\left(\dfrac{1}{e}\right)^{a+1}\right]=\ln \left(\dfrac{1}{e}\right)^{a+1}-\left(\dfrac{1}{e}\right)^{a+1}+a\)\(=-a-1-\left(\dfrac{1}{e}\right)^{a+1}+a=-1-\left(\dfrac{1}{e}\right)^{a+1}<0\),
易證\(x>1\)時,\(e^x>2x\),
所以\(a>1\),\(f'(e^a )=2a-e^a<0\),
故存在\(x_1\) ,\(x_2\)滿足\(0<\left(\dfrac{1}{e}\right)^{a+1}<x_1<1<x_2<e^a\),\(f'(x_1)=f'(x_2)=0\),
當\(x\in (0 ,x_1)\)時,\(f(x)\)單調遞減,
當\(x\in (x_1 ,x_2)\)時,\(f(x)\)單調遞增,
當\(x\in (x_2 ,+∞)\)時,\(f(x)\)單調遞減,
所以\(f(x)\)在\(x=x_1\)處有極小值,在\(x=x_2\)處有極大值.
綜上所述,當\(a≤1\)時,\(f(x)\)沒有極值點;當\(a>1\)時,\(f(x)\)有\(2\)個極值點.
點撥
① 求出導函式\(f'(x)=\ln x-x+a\),它的圖象很難確定,不知道是否存在零點(這與原函式單調性有關),則考慮二次求導進行分析;
② 當\(a>1\)時,導函式\(f'(x)=\ln x-x+a\)存在零點\(x_1\) ,\(x_2\)是怎麼確定的?
誤區1:\(y=f'(x)\)最大值在x軸上方且是“先增後減”,想當然說它有兩個零點是不嚴謹的.因為\(y=f'(x)\)的圖象可能如下左圖,則只有一個零點;如右圖,甚至沒有零點;
誤區2:當\(x→0\)時,顯然\(f'(x)→-∞\),當\(x→+∞\)時,顯然\(f'(x)→-∞\),
那可知\(y=f'(x)\)存在兩個零點,也不夠嚴謹;
而因\(f^{\prime}\left[\left(\dfrac{1}{e}\right)^{a+1}\right]<0\), \(f'(e^a)=2a-e^a<0\),
由零點判定定理可確定\(y=f'(x)\)有兩個零點\(x_1\)、\(x_2\).
③ 那“取點”\(\left(\dfrac{1}{e}\right)^{a+1}\)、\(e^a\)是怎麼想到的呢?這需要些技巧,導函式\(f'(x)=\ln x-x+a\)中有引數\(a>1\),\(x\)取常數是不行的;因有\(\ln x\),想到含\(a\)的\(e\)指數冪,多嘗試就可以!
【典題2】已知函式\(f(x)=\dfrac{1}{x}+a\ln x-a-1\),\(a\in R\).
(1)討論函式\(f(x)\)的單調性;(2)討論函式\(f(x)\)的零點個數.
解析 (1) \(f^{\prime}(x)=-\dfrac{1}{x^2}+\dfrac{a}{x}=\dfrac{a x-1}{x^2}\),
當\(a≤0\)時,\(f'(x)<0\),故\(f(x)\)在\((0,+∞)\)上單調遞減,
當\(a>0\)時, \(f^{\prime}(x)>0 \Rightarrow x>\dfrac{1}{a}\),
\(f(x)\)在\(\left(0, \dfrac{1}{a}\right)\)上單調遞減,在\(\left(\dfrac{1}{a},+\infty\right)\)上單調遞增.
(2)當\(a≤0\)時, \(f\left(\dfrac{1}{e}\right)=e-2 a-1 \geq e-1>0\),\(f(e)=\dfrac{1}{e}-1<0\),\(f(x)\)有唯一零點;
當\(a>0\)時,\(f(1)=-a<0\),
(不用最小值\(f\left(\dfrac{1}{a}\right)=-a\ln a-1<0\),因為這個還要證明,用\(f(1)=-a<0\)容易,接著要在\(x=1\)兩邊各取點\(0<x_1<1\),\(x_2>1\)使得\(f(x_1 )>0\),\(f(x_2 )>0\), )
而\(e^{1+\dfrac{1}{a}}>1,\), \(f\left(e^{1+\dfrac{1}{a}}\right)=\dfrac{1}{e^{1+\dfrac{1}{a}}}+a\left(1+\dfrac{1}{a}\right)-a-1=\dfrac{1}{e^{1+\dfrac{1}{a}}}>0\),(取 \(x_2=e^{1+\dfrac{1}{a}}\))
\(\therefore f(x)\)在 \(\left(1, e^{1+\dfrac{1}{a}}\right)\)內有一個零點;
\(f\left(e^{-a-1}\right)=e^{a+1}-(a+1)^2\),(取 \(x_1=e^{-a-1}<1\))
令\(g(t)=e^t-t^2\),則\(g'(t)=e^t-2t\),\(g'' (t)=e^t-2\),
當\(t>1\)時,\(g'' (t)>e-2>0\),\(\therefore g'(t)\)單調遞增,
\(\therefore g'(t)>g'(1)=e-2>0\),\(\therefore g(t)\)單調遞增,
\(\therefore g(t)>g(1)=e-1>0\),故 \(f\left(e^{-a-1}\right)>0\),
\(\because e^{-a-1}<1\),\(\therefore f(x)\)在\((e^{-a-1},1)\)內有一個零點;
\(\therefore\)當\(a>0\)時\(f(x)\)有兩個零點.
綜上,當\(a≤0\)時\(f(x)\)有一個零點,當\(a>0\)時\(f(x)\)有兩個零點.
【鞏固練習】
- 若函式\(f(x)=-\dfrac{1}{3} x^3+a x^2+3 a^2 x-\dfrac{5}{3}\)僅有一個零點,求實數\(a\)的取值範圍.
2.討論函式\(f(x)=2 e^{2 x}-\dfrac{a}{x}\),\(x\in (0,1)\)的零點的個數.
3.已知函式\(f(x)=ax⋅\ln x\)(其中\(a≠0\),\(a\in R\)), \(g(x)=\dfrac{x-1}{x+1}\).
(1)若存在實數\(a\)使得\(f(x)<\dfrac{1}{e}\)恆成立,求\(a\)的取值範圍;
(2)當\(a⩽\dfrac{1}{2}\)時,討論函式\(y=f(x)-g(x)\)的零點個數.
參考答案
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答案 \(\left(-1, \dfrac{\sqrt[3]{5}}{3}\right)\)
解析 函式\(f(x)\)只有一個零點,
因為\(f'(x)=-x^2+2ax+3a^2=-(x-3a)(x+a)\),
①當\(a<0\)時,由\(f'(x)>0\),解得\(3a<x<-a\),
所以函式\(f(x)\)在區間\((3a,-a)\)上單調遞增,
由\(f'(x)<0\),解得\(x<3a\)或\(x>-a\),
所以函式\(f(x)\)在區間\((-∞,3a)\),\((-a,+∞)\)上單調遞減,
又\(f(0)=-\dfrac{5}{3}<0\),
所以只需要\(f(-a)<0\),解得\(-1<a<0\),
所以實數\(a\)的取值範圍為\((-1,0)\).
②當\(a=0\)時,顯然\(f(x)\)只有一個零點成立,
③當\(a>0\)時,由\(f'(x)>0\),解得\(-a<x<3a\),
即\(f(x)\)在區間\((-a,3a)\)上單調遞增,
由\(f'(x)<0\),解得\(x<-a\)或\(x>3a\),
即函式\(f(x)\)在區間\((-∞,-a)\),\((3a,+∞)\)上單調遞減,
又\(f(0)=-\dfrac{5}{3}<0\),
所以只需\(f(3a)<0\),解得 \(0<a<\dfrac{\sqrt[3]{5}}{3}\),
綜上,實數\(a\)的取值範圍為\(\left(-1, \dfrac{\sqrt[3]{5}}{3}\right)\). -
答案 當\(a≤0\)或\(a≥2e^2\)時,\(f(x)\)無零點;當\(0<a<2e^2\)時,\(f(x)\)存在\(1\)個零點 .
解析 \(f^{\prime}(x)=4 e^{2 x}+\dfrac{a}{x^2}\)
①當\(a≤0\)時,\(f(x)>0\)恆成立,在\((0,1)\)上恆成立,\(\therefore f(x)\)在\((0,1)\)上無零點;
②當\(a≥2e^2\)時, \(\because f^{\prime}(x)=4 e^{2 x}+\dfrac{a}{x^2}>0\),在\((0,1)\)上恆成立,
\(\therefore f(x)\)在\((0,1)\)上單調遞增,
\(\therefore f(x)<f(1)=2e^2-a<0\),\(\therefore f(x)\)在\((0,1)\)上無零點;
③當\(0<a<2e^2\)時, \(\because f^{\prime}(x)=4 e^{2 x}+\dfrac{a}{x^2}>0\)在\((0,1)\)上恆成立,
\(\therefore f(x)\)在\((0,1)\)上單調遞增.
又\(\because\)當\(x\)趨向於\(0\)時,\(f(x)\)趨向於\(-∞\);且\(f(1)=2e^2-a>0\).
故由零點存在性定理可知:\(f(x)\)在\((0,1)\)上存在唯一一個零點,
綜上:當\(a≤0\)或\(a≥2e^2\)時,\(f(x)\)在\((0,1)\)上無零點;
當\(0<a<2e^2\)時,\(f(x)\)在\((0,1)\)上存在唯一一個零點 . -
答案 (1)\((-1,0)\);
(2) 當\(a<0\)或\(a=\dfrac{1}{2}\)時,\(h(x)\)有\(1\)個零點,當\(0<a<\dfrac{1}{2}\)時,\(h(x)\)有\(2\)個零點.
解析 (1)因為\(f(x)=ax\ln x\),\(a≠0\),要使得\(f(x)<\dfrac{1}{e}\)在\((0,+∞)\)上恆成立,
所以\(a<0\),由\(f'(x)=a(\ln x+1)\),
由\(f'(x)=a(\ln x+1)>0\),解得\(0<x<\dfrac{1}{e}\),
由\(f'(x)=a(\ln x+1)<0\),解得\(x>\dfrac{1}{e}\) ,
所以\(f(x)_{\max }=f\left(\dfrac{1}{e}\right)=-\dfrac{a}{e}\),
所以\(-\dfrac{a}{e}<\dfrac{1}{e}\),所以\(-1<a<0\),
所以\(a\)的取值範圍為\((-1,0)\).
(2)①當\(a<0\)時,當\(x\in (0,1)\)時,\(f(x)>0\),\(g(x)<0\),
所以\(y=f(x)-g(x)\)恆大於零,
當\(x=1\)時,\(y=f(x)-g(x)=0\),
令\(h(x)=f(x)-g(x)\),
所以\(a<0\)時,令\(h(x)\)在\((0,+∞)\)只有\(1\)個零點,
②當\(a>0\)時,令\(h(x)=f(x)-g(x)\),
則\(h(x)=a x \ln x-1+\dfrac{2}{x+1}(x>0)\),
\(h^{\prime}(x)=a(\ln x+1)-\dfrac{2}{(x+1)^2}\), \(h^{\prime \prime}(x)=\dfrac{a}{x}+\dfrac{4}{(x+1)^3}\),
因為\(x>0\),所以\(h'' (x)>0\)恆成立,
所以\(h'(x)\)在\((0,+∞)\)上單調遞增,
因為\(h(1)=0\),當\(h'(1)=0\),即\(a=\dfrac{1}{2}\)時,
\(h'(x)\)在\((0,1)\)上恆小於零,在\((1,+∞)\)上恆大於零,
即 在\((0,1)\)上單調遞減,在\((1,+∞)\)上單調遞增,
所以\(h(x)⩾h(1)=0\),\(y=h(x)\)在\((0,+∞)\)只有\(1\)個零點,
若\(0<a<\dfrac{1}{2}\)時,\(h'(1)=a-\dfrac{1}{2}<0\),
由於\(h'(x)\)在\((0,+∞)\)上單調遞增,
所以\(h'(x)\)在\((0,1]\)上恆小於零,\(h(x)\)在\((0,1]\)上單調遞減,
因為\(h(1)=0\),所以\(h(x)\)在\((0,1]\)上有唯一零點\(1\),
又因為\(h'(1)=a-\dfrac{1}{2}<0\), \(h^{\prime}\left(e^{\dfrac{2}{a}-1}\right)=2-\dfrac{2}{\left(e^{\dfrac{2}{a}-1}+1\right)^2}>0\),
所以存在 \(x_0 \in\left(1, e^{\dfrac{2}{a}-1}\right)\),使得\(h'(x_0 )=0\),
由於\(h'(x)\)在\((0,+∞)\)上單調遞增, (1) ,
所以\(h(x)\)在\((1,x_0 )\)上單調遞減,在\((x_0,+∞)\)上單調遞增, \(x_0 \in\left(1, e^{\dfrac{2}{a}-1}\right)\),
所以\(h(x_0 )<h(1)=0\),
又\(0<a<\dfrac{1}{2}\), \(e^{\dfrac{1}{a}}>1\), \(h\left(e^{\dfrac{1}{a}}\right)=e^{\dfrac{1}{a}}-1+\dfrac{2}{e^{\dfrac{1}{a}+1}}>0\),
所以 \(x_0<e^{\dfrac{1}{a}}\),
結合\(h(x)\)在\((x_0,+∞)\)單調遞增,\(h(x)\)在\((1,+∞)\)上有唯一零點,
又\(h(1)=0\),
所以\(0<a<\dfrac{1}{2}\)時,\(h(x)\)在(0,+∞)上有唯一零點,
又因為\(h(1)=0\),
所以\(0<a<\dfrac{1}{2}\)時,\(h(x)\)在\((0,+∞)\)上有\(2\)個零點,
綜上所述,當\(a<0\)或\(a=\dfrac{1}{2}\)時,\(h(x)\)在\((0,+∞)\)只有\(1\)個零點,
當\(0<a<\dfrac{1}{2}\)時,\(h(x)\)在\((0,+∞)\)上有\(2\)個零點.
【題型4】幾何法
【典題1】 函式\(g(x)=(x^2+1) e^x-mx-1\)在\([-1,+∞)\)有兩個零點,求\(m\)的取值範圍.
解析 函式\(g(x)=(x^2+1) e^x-mx-1\)在\([-1,+∞)\)有兩個零點,
等價於\(s(x)=(x^2+1) e^x\)與\(t(x)=mx+1\)在\([-1,+∞)\)有兩個交點,
\(\because s'(x)=(x+1)^2 e^x≥0\),\(\therefore s(x)\)在\([-1,+∞)\)單調遞增,
又 \(s(-1)=\dfrac{2}{e}\),\(s(0)=1\),
而函式\(t(x)=mx+1\)是過定點\((0,1)\)的直線,且斜率為\(m\),
則 \(\dfrac{1-\dfrac{2}{e}}{0-(-1)}=1-\dfrac{2}{e} \leq m<1\),或\(m>s'(0)=1\),
故m的取值範圍為 \(1-\dfrac{2}{e} \leqslant m<1\)或\(m>1\).
點撥 函式能夠轉化為一次函式與其他形式函式的交點問題,可考慮這種方法;但還要注意到\(s(x)=(x^2+1) e^x\)是凹函式,因為\(s'' (x)>0\),若\(s(x)\)是凸函式,則\(m\)的範圍就不一樣了,但內容超綱不適合處理解答題,第二問利用分離引數法有難度 \(m=\dfrac{\left(x^2+1\right) e^x-1}{x}\),出現洛必達法則、隱零點.
【鞏固練習】
1.函式 \(F(x)=\dfrac{a x-a}{e^x}+1(a<0)\)沒有零點,求實數\(a\)的取值範圍.
參考答案
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答案 \((-e^2,0)\)
解析 由已知有 \(F(x)=\dfrac{a x-a+e^x}{e^x}=0\)沒有解,即\(e^x=-a(x-1)\)無解,
\(\therefore y_1=e^x\)與\(y_2=-a(x-1)\)兩圖象無交點,
設兩圖象相切於\((m,n)\)兩點,
\(\therefore\left\{\begin{array}{l} e^m=-a(m-1) \\ e^m=-a \end{array}\right.\),\(\therefore m=2\),\(a=-e^2\),
\(\because\)兩圖象無交點,\(\therefore\)故\(a\)的取值範圍是\((-e^2,0)\).
分層練習
【A組---基礎題】
1.已知函式\(f(x)=ax^3+bx^2+cx\)在點\(x_0\)處取得極小值\(-4\),使其導數\(f'(x)>0\)的\(x\)的取值範圍為\((1,3)\),
(1)求\(f(x)\)的解析式;
(2)若過點\(P(-1,m)\)作曲線\(y=f(x)\)的三條切線,求實數\(m\)的取值範圍.
2.證明:\(a>1\), \(f(x)=(1+x^2 ) e^x-a\)在 \((-∞,+∞)\)上有且僅有一個零點.
3.已知函式 \(f(x)=\dfrac{1}{3} x^3-\dfrac{(k+1)}{2} x^2\),\(g(x)=\dfrac{1}{3}-kx\),且\(f(x)\)在區間\((2,+∞)\)上為增函式.
(1)求實數\(k\)的取值範圍;
(2)若函式\(f(x)\)與\(g(x)\)的圖象有三個不同的交點,求實數\(k\)的取值範圍.
4.已知函式\(f(x)=ax^2+x-\ln x(a\in R)\).
(1)當\(a=1\)時,求\(f(x)\)在區間 \(\left[\dfrac{1}{3}, 1\right]\)上的最值;
(2)若\(g(x)=f(x)-x\)在定義域內有兩個零點,求\(a\)的取值範圍.
5.若 \(f(x)=e^{x-2}-a x\)有兩個零點,求實數\(a\)的取值範圍.
6.已知函式\(f(x)=x^2-a\ln x\),
(1)求\(f(x)\)的單調區間;
(2)如果\(a>0\),討論函式\(y=f(x)\)在區間\((1,e)\)上零點的個數.
7.已知函式\(f(x)=x^3+ax+\dfrac{1}{4}\),\(g(x)=-\ln x\).用\(\min \{m,n\}\)表示\(m\),\(n\)中的最小值,設函式 \(h(x)=\min \{f(x), g(x)\}(x>0)\),討論\(h(x)\)零點的個數 .
8.若\(f(x) =m^2 x^2-\ln x\)的圖象與直線\(y=mx\)交於\(M(x_M,y_M )\),\(N(x_N,y_N )\),兩點,且\(x_M>x_N>1\),求實數\(m\)的取值範圍.
9.已知\(g(x)=a^x+b^x-2(0<a<1,b>1)\)有且只有\(1\)個零點,求\(ab\)的值.
10.已知\(a>0\),函式\(f(x)=ae^x-x-2\),函式\(g(x)=\ln (x+2)-x-\ln a\).
(1)若對\(∀x\in R\),\(f(x)⩾0\)恆成立,求\(a\)的取值範圍;
(2)若方程\(f(x)=g(x)\)有兩個根,求\(a\)的取值範圍.
參考答案
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答案 (1)\(f(x)=-x^3+6x^2-9x\);(2) \((-11,16)\) .
解析 (1)\(f(x)=-x^3+6x^2-9x\)(過程略).
(2)設切點\((t,f(t))\),\(y-f(t)=f'(t)(x-t)\)
\(y=(-3t^2+12t-9)(x-t)+(-t^3+6t^2-9t)\)
\(=(-3t^2+12t-9)x+t(3t^2-12t+9)-t(t^2-6t+9)\)
\(=(-3t^2+12t-9)x+t(2t^2-6t)\)過\((-1,m)\)
\(\therefore m=(-3t^2+12t-9)(-1)+2t^3-6t^2\),
\(\therefore 2t^3-3t^2-12t+9-m=0\),
設\(g(t)=2t^3-3t^2-12t+9-m\),
令\(g'(t)=6t^2-6t-12=6(t^2-t-2)=0\),
求得\(t=-1\),\(t=2\),
若要方程\(g(t)=0\)有三個根,
需\(\left\{\begin{array} { l } { g ( - 1 ) > 0 } \\ { g ( 2 ) < 0 } \end{array} \Rightarrow \left\{\begin{array} { l } { - 2 - 3 + 1 2 + 9 - m > 0 } \\ { 1 6 - 1 2 - 2 4 + 9 - m < 0 } \end{array} \Rightarrow \left\{\begin{array}{l} m<16 \\ m>-11 \end{array}\right.\right.\right.\)
故\(-11<m<16\);因此所求實數\(m\)的範圍為:\((-11,16)\). -
證明 \(f'(x)=e^x (x^2+2x+1)=e^x (x+1)^2\),\(\therefore f'(x)≥0\),
\(\therefore f(x)=(1+x^2 ) e^x-a\)在\((-∞,+∞)\)上為增函式.
而\(f(0)=1-a<0\), \(f(\sqrt{a})=(1+a) e^{\sqrt{a}}-a=e^{\sqrt{a}}+a\left(e^{\sqrt{a}}-1\right)>0\),
\(\therefore f(x)\)在\((-∞,+∞)\)上有且只有一個零點. -
答案 (1) \(k≤1\);(2) \(k<1-\sqrt{3}\).
解析 (1)\(k≤1\)(過程略).
(2)欲使\(f(x)\)與\(g(x)\)的圖象有三個不同的交點,
即函式\(h(x)=f(x)-g(x)\)有三個不同的零點,
設 \(h(x)=f(x)-g(x)=\dfrac{x^3}{3}-\dfrac{(k+1)}{2} x^2+k x-\dfrac{1}{3}\),
\(h'(x)=x^2-(k+1)x+k=(x-k)(x-1)\),
令\(h'(x)=0\)得\(x=k\)或\(x=1\),
由(1)知\(k≤1\),
①當\(k=1\)時,\(h'(x)=(x-1)^2≥0\),\(h(x)\)在\(R\)上遞增,顯然不合題意.
②當\(k<1\)時,\(h(x)\),\(h'(x)\)隨x的變化情況如下表:
\(x\) | \((-∞,k)\) | \(k\) | \((k,1)\) | \(1\) | \((1,+∞)\) |
---|---|---|---|---|---|
\(h'(x)\) | \(+\) | \(0\) | \(-\) | \(0\) | \(+\) |
\(h(x)\) | \(↗\) | 極大值 | |||
\(-\dfrac{k^3}{6}+\dfrac{k^2}{2}-\dfrac{1}{3}\) | \(↘\) | 極小值 | |||
\(\dfrac{k-1}{2}\) | \(↗\) |
由於 \(h(1)=\dfrac{k-1}{2}<0\)且當\(x→+∞\)時,\(y→+∞\),當\(x→-∞\)時\(,y→-∞\),
若要函式\(h(x)=f(x)-g(x)\)有三個不同的零點,
故需 \(-\dfrac{k^3}{6}+\dfrac{k^2}{2}-\dfrac{1}{3}>0\),即\((k-1)(k^2-2k-2)<0\),
\(\therefore\left\{\begin{array}{l}
k<1 \\
k^2-2 k-2>0
\end{array}\right.\),解得 \(k<1-\sqrt{3}\),
綜上,所求\(k\)的取值範圍為\(k<1-\sqrt{3}\).
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答案 (1)\(f(x)_{min}=\ln 2+\dfrac{3}{4}\),\(f(x)_{max}=2\);(2) \(\left(0, \dfrac{1}{2 e}\right)\).
解析 (1) \(f(x)_{min}=\ln 2+\dfrac{3}{4}\),\(f(x)_{max}=2\)(過程略)
(2)令\(g(x)=f(x)-x=0\),得 \(a=\dfrac{\ln x}{x^2}\),
設 \(h(x)=\dfrac{\ln x}{x^2}(x>0)\),
\(\because g(x)=f(x)-x\)在定義域內有兩個零點,
\(\therefore\)函式\(y=h(x)\)與\(y=a\)與在定義域內有兩個交點
\(\because h^{\prime}(x)=\dfrac{1-2 \ln x}{x^3}\) ,
令\(h'(x)>0\)得:\(0<x<\sqrt{e}\);令\(h'(x)<0\)得:\(x>\sqrt{e}\),
\(\therefore h(x)\)在\((0,\sqrt{e})\)單調遞增,在\((\sqrt{e},+∞)\)單調遞減,
又\(\because h(\sqrt{e})=\dfrac{1}{2e}\),且當\(x→0\)時,\(h(x)→-∞\),當\(x→+∞\)時,\(h(x)→0\),
畫出函式\(h(x)\)的大致影象,如圖所示:
由圖象可得, \(a \in\left(0, \dfrac{1}{2 e}\right)\),
\(\therefore a\)的取值範圍為 \(\left(0, \dfrac{1}{2 e}\right)\). -
答案 \(\left(\dfrac{1}{e},+\infty\right)\).
解析 方法1
因為\(f(x)=e^{x-2}-ax\),所以 \(f^{\prime}(x)=e^{x-2}-a\),
當\(a⩽0\)時,\(f'(x)>0\),\(f(x)\)在\(R\)上單調遞增,
故\(f(x)\)至多存在一個零點,不合題意;
當\(a>0\)時,由\(f'(x)=e^{x-2}-a=0\)得:\(x=2+\ln a\),
當\(x\in (-∞,2+\ln a)\)時,\(f'(x)<0\),\(f(x)\)在\((-∞,2+\ln a)\)上單調遞減;
當\(x\in (2+\ln a,+∞)\)時,\(f'(x)>0\),\(f(x)\)在\((2+\ln a,+∞)\)上單調遞增;
所以當\(x=2+\ln a\)時,\(f(x)\)取到最小值,且最小值為\(f(2+\ln a)=-a(1+\ln a)\).
①當\(0<a⩽\dfrac{1}{e}\)時,\(f(2+\ln a)⩾0\),\(f(x)\)在\(R\)上至多存在一個零點,不合題意;
②當\(a>\dfrac{1}{e}\)時,\(f(2+\ln a)<0\).
由於 \(f(0)=e^{-2}>0\),
所以\(f(x)\)在\((-∞,2+\ln a)\)上存在唯一零點.
\(f(4+2 \ln a)=e^{2 \ln a+2}-a(4+2 \ln a)=e^2 a^2-a(4+2 \ln a)=a\left(e^2 a-2 \ln a-4\right)\).
設\(g(a)=e^2 a-2\ln a-4\),
則 \(g^{\prime}(a)=e^2-\dfrac{2}{e}=\dfrac{e^2 a-2}{a}=\dfrac{e^2}{a}\left(a-\dfrac{2}{e^2}\right)\),
當\(a>\dfrac{1}{e}\)時,\(g'(a)>0\),
所以\(g(a)\)在\(\left(\dfrac{1}{e},+\infty\right)\)上單調遞增.
因為\(g\left(\dfrac{1}{e}\right)=e+2-4>0\),
所以\(g(a)>g\left(\dfrac{1}{e}\right)>0\),即\(f(4+2\ln a)>0\).
從而\(f(x)\)在\(R\)上有兩個零點.
綜上所述,\(a\)的取值範圍為\(\left(\dfrac{1}{e},+\infty\right)\).
方法2 幾何法
函式\(f(x)=e^{x-2}-ax\)有兩個零點等價於函式\(g(x)=e^{x-2}\)與函式\(y=ax\)有兩個交點,
設函式\(y=kx\)與函式\(g(x)=e^{x-2}\)相切,切點\(P(x_0,y_0)\),
\(\because g'(x)=e^{x-2}\),\(\therefore k=g'(x_0 )=e^{x_0-2}\),
即切線方程為\(y=e^{x_0-2} x\),則\(y_0=e^{x_0-2} x_0\),
又\(g(x_0 )=e^{x_0-2}\),即\(y_0=e^{x_0-2}\),\(\therefore x_0=1\),
\(\therefore k=e^{x_0-2}=\dfrac{1}{e}\),
\(\therefore\)若函式\(g(x)=e^{x-2}\)與函式\(y=ax\)有兩個交點,則\(a>\dfrac{1}{e}\),
所以\(a\)的取值範圍為 \(\left(\dfrac{1}{e},+\infty\right)\). -
答案 (1)當\(a≤0\),\(f(x)\)在\((0,+∞)\)上遞增;
當\(a>0\),\(f(x)\)的增區間為\(\left(\sqrt{\dfrac{a}{2}},+\infty\right)\),減區間為\(\left(0, \sqrt{\dfrac{a}{2}}\right)\);
(2)當\(0<a<2e\)時,函式\(f(x)\)無零點;
當\(a=2e\)或\(a≥e^2\)時,函式\(f(x)\)有\(1\)零點;
當\(2e<a<e^2\)時,函式\(f(x)\)有\(2\)個零點.
解析(1)函式\(f(x)=x^2-a\ln x\)的導數 \(f^{\prime}(x)=2 x-\dfrac{a}{x}=\dfrac{2 x^2-a}{x}(x>0)\),
若\(a≤0\),則\(f'(x)>0\),即有\(f(x)\)在\((0,+∞)\)上遞增;
若\(a>0\),由\(f'(x)>0\)得到\(x>\sqrt{\dfrac{a}{2}}\),由\(f'(x)<0\)得到\(0<x<\sqrt{\dfrac{a}{2}}\),
即有\(a>0\)時,\(f(x)\)的增區間為\(\left(\sqrt{\dfrac{a}{2}},+\infty\right)\),減區間為\(\left(0, \sqrt{\dfrac{a}{2}}\right)\);
(2)由(1)知\(f(x)\)的極小值為 \(f\left(\sqrt{\dfrac{a}{2}}\right)=\dfrac{a}{2}\left(1-\ln \dfrac{a}{2}\right)\),也為最小值.
① 當\(\dfrac{a}{2}\left(1-\ln \dfrac{a}{2}\right)>0\),即\(0<a<2e\),\(f(x)\)的最小值大於\(0\),
則\(y=f(x)\)在區間\((1,e)\)上無零點;
② 當\(\dfrac{a}{2}\left(1-\ln \dfrac{a}{2}\right)=0\),即\(a=2e\),即有\(1<\sqrt{\dfrac{a}{2}}<e\),
而\(f(1)=1>0\), \(f\left(\sqrt{\dfrac{a}{2}}\right)=0\),\(f(e)>0\),
則\(f(x)\)在\((1,e)\)上有一個零點;
③ 當\(\dfrac{a}{2}\left(1-\ln \dfrac{a}{2}\right)<0\),即\(a>2e\),即有\(\sqrt{\dfrac{a}{2}}>\sqrt{e}>1\),
(i)若\(\sqrt{\dfrac{a}{2}}<e\),即\(2e<a<2e^2\),而\(f(e)=e^2-a\),
當\(2e<a<e^2\)時,\(f(e)≥0\),則\(f(x)\)在\((1,e)\)上有\(2\)零點.
當\(e^2≤a<2e^2\)時,\(f(e)≤0\),則\(f(x)\)在\((1,e)\)上有\(1\)個零點;
(ii)若\(\sqrt{\dfrac{a}{2}}≥e\),即\(a≥2e^2\),而\(f(1)>0\),\(f(e)<0\),
則\(f(x)\)在\((1,e)\)上有\(1\)零點.
綜上所述:當\(0<a<2e\)時,函式\(f(x)\)無零點;
當\(a=2e\)或\(a≥e^2\)時,函式\(f(x)\)有\(1\)零點;
當\(2e<a<e^2\)時,函式\(f(x)\)有\(2\)個零點. -
答案 當\(a>-\dfrac{3}{4}\),\(h(x)\)有\(0\)個零點,
當 \(a=-\dfrac{3}{4}\)或\(a≤-\dfrac{5}{4}\)時,\(h(x)\)有一個零點,
當\(-\dfrac{5}{4}<a<-\dfrac{3}{4}\)時,\(h(x)\)有兩個零點.
解析 當 \(x\in (1,+∞)\)時,\(g(x)=-\ln x<0\),
\(\therefore\) 函式 \(h(x)=\min \{f(x), g(x)\} \leq g(x)<0\),
故\(h(x)\)在\((1,+∞)\)時無零點.
當\(x=1\)時,若\(a≥-\dfrac{5}{4}\),則\(f(1)=a+\dfrac{5}{4}≥0\),
\(\therefore h(x)=\min \{f(1), g(1)\}=g(1)=0\),
故\(x=1\)是函式\(h(x)\)的一個零點;
若\(a<-\dfrac{5}{4}\),則\(f(1)=a+\dfrac{5}{4}<0\),
\(\therefore h(x)=\min \{f(1), g(1)\}=f(1)<0\),
故\(x=1\)不是函式\(h(x)\)的零點;
當 \(x\in (0,1)\)時,\(g(x)=-\ln x>0\),
所以只需考慮\(f(x)\)在\((0,1)\)的零點個數,
(i)若\(a≤-3\)或\(a≥0\),則\(f'(x)=3x^2+a\),
當\(a≥0\)時,\(f(x)\)在\((0,1)\)無零點,
當\(a≤-3\)時,\(f(x)\)在\((0,1)\)單調,
而\(f(0)=\dfrac{1}{4}\),\(f(1)=a+\dfrac{5}{4}\),\(f(x)\)在\((0,1)\)有一個零點,
(ii)若\(-3<a<0\),
則\(f(x)\)在\(\left(0, \sqrt{-\dfrac{a}{3}}\right)\)單調遞減,在\(\left(\sqrt{-\dfrac{a}{3}}, 1\right)\)單調遞增,
故當 \(x=\sqrt{-\dfrac{a}{3}}\)時,\(f(x)\)取得最小值,最小值為 \(f\left(\sqrt{-\dfrac{a}{3}}\right)=\dfrac{2 a}{3} \sqrt{-\dfrac{a}{3}}+\dfrac{1}{4}\),
①若\(f\left(\sqrt{-\dfrac{a}{3}}\right)>0\),即\(-\dfrac{3}{4}<a<0\),\(f(x)\)在\((0,1)\)無零點,
②若\(f\left(\sqrt{-\dfrac{a}{3}}\right)=0\),即\(a=-\dfrac{3}{4}\),則\(f(x)\)在\((0,1)\)有唯一零點,
③若\(f\left(\sqrt{-\dfrac{a}{3}}\right)<0\),即\(-3<a<-\dfrac{3}{4}\),由於\(f(0)=\dfrac{1}{4}\),\(f(1)=a+\dfrac{5}{4}\),
所以當\(-\dfrac{5}{4}<a<-\dfrac{3}{4}\)時,\(f(x)\)在\((0,1)\)有兩個零點,
當\(-3<a≤-\dfrac{5}{4}\)時,\(f(x)\)在\((0,1)\)有一個零點,
綜上所述,當\(a>-\dfrac{3}{4}\),\(h(x)\)有\(0\)個零點,
當 \(a=-\dfrac{3}{4}\)或\(a≤-\dfrac{5}{4}\)時,\(h(x)\)有一個零點,
當\(-\dfrac{5}{4}<a<-\dfrac{3}{4}\)時,\(h(x)\)有兩個零點. -
答案 \(\left(-\dfrac{1}{2 e^{\dfrac{3}{4}}}, 0\right)\)
解析 令 \(m^2 x^2-\ln x=mx\) ,
則由題意可知\(m^2 x^2-\ln x-mx=0\)有兩個大於\(1\)的實數根,顯然\(m≠0\).
令\(F(x)=m^2 x^2-\ln x-mx\),
則 \(F^{\prime}(x)=2 m^2 x-\dfrac{1}{x}-m=\dfrac{(2 m x+1)(m x-1)}{x}\).
若\(m>0\),則當 \(x \in\left(0, \dfrac{1}{m}\right)\)時,\(F'(x)<0\),
當 \(x \in\left(-\dfrac{1}{2 m}, \quad+\infty\right)\)時,\(F'(x)>0\),
要滿足已知條件,必有 \(\left\{\begin{array}{c} F(1)=m^2-m>0 \\ F\left(\dfrac{1}{m}\right)=-\ln \dfrac{1}{m}<0 \\ \dfrac{1}{m}>1 \end{array}\right.\)此時無解;
若\(m<0\),則當 \(x \in\left(0,-\dfrac{1}{2 m}\right)\)時,\(F'(x)<0\),
當 \(x \in\left(-\dfrac{1}{2 m},+\infty\right)\)時,\(F'(x)>0\),
要滿足已知條件,必有\(\left\{\begin{array}{c} F(1)=m^2-m>0, \\ F\left(-\dfrac{1}{2 m}\right)=\dfrac{3}{4}+\ln (-2 m)<0 \\ -\dfrac{1}{2 m}>1 \end{array}\right.\),解得\(-\dfrac{1}{2 e^{\dfrac{3}{4}}}<m<0\).
當\(-\dfrac{1}{2 e^{\dfrac{3}{4}}}<m<0\)時,\(F(x)\)在\(\left(1,-\dfrac{1}{2 m}\right)\)上單調遞減,\(F(1) \cdot F\left(-\dfrac{1}{2 m}\right)<0\),
故函式\(F(x)\)在\(\left(1,-\dfrac{1}{2 m}\right)\)上有一個零點.
易知 \(\dfrac{1}{m^2}>-\dfrac{1}{2 m}\),且 \(F\left(\dfrac{1}{m^2}\right)=\dfrac{1}{m^2}-\dfrac{1}{m}-\ln \dfrac{1}{m^2}>\dfrac{1}{m^2}-\ln \dfrac{1}{m^2}\) ,
下證:\(x-\ln x>0\).
令\(g(x)=x-\ln x\),則\(g'(x)=1-\dfrac{1}{x}\),當\(0<x<1\)時\(g'(x)<0\),
當\(x>1\)時,\(g'(x)>0\),
故\(g(x)≥g(1)=1-\ln 1>0\),即\(x-\ln x>0\),
故\(F\left(\dfrac{1}{m^2}\right)>\dfrac{1}{m^2}-\ln \dfrac{1}{m^2}>0\),
故\(F\left(-\dfrac{1}{2 m}\right) \cdot F\left(\dfrac{1}{m^2}\right)<0\),
又\(F(x)\)在 \(\left(-\dfrac{1}{2 m},+\infty\right)\)上單調遞增,
故\(F(x)\)在 \(\left(-\dfrac{1}{2 m},+\infty\right)\)上有一個零點.
綜上所述,實數\(m\)的取值範圍為 \(\left(-\dfrac{1}{2 e^{\dfrac{3}{4}}}, 0\right)\). -
答案 \(1\)
解析 \(\because g(0)=0\),由題意知\(0\)為\(g(x)=f(x)-2\)的唯一零點,
因為\(g'(x)=a^x \ln a+b^x \ln b\),且\(0<a<1\),\(b>1\),
故\(\ln a<0\),\(\ln b>0\),
所以\(g'(x)=0\)有唯一解 \(x_0=\log _{\dfrac{b}{a}}\left(-\dfrac{\ln a}{\ln b}\right)\),
令\(h(x)=g'(x)\),則\(h'(x)=(a^x \ln a+b^x \ln b)'=a^x (\ln a)^2+b^x (\ln b)^2\),
對於任意\(x\in R\),都有\(h'(x)>0\),
故\(g'(x)=h(x)\)在\(R\)上單調遞增,
則\(x\in (-∞,x_0 )\)時,\(g'(x)<0\),\(x\in (x_0,+∞)\)時,\(g'(x)>0\),
故函式\(g(x)\)在\((-∞,x_0 )\)時單調遞減,在\((x_0,+∞\))時單調遞增,
故\(g(x)_{min}=g(x_0 )\);
若\(g(x_0 )<0\),當\(x<\log _a2\)時, \(a^x>a^{\log _a^2}=2\),\(b^x>0\),
則\(g(x)>0\),因此當\(x_1<\log _a2\)且\(x_1<x_0\)時,\(g(x_1 )>0\),
此時\(g(x)\)在\((x_1,x_0 )\)內有零點,則\(g(x)\)至少有兩個零點,與題意不符;
故\(g(x_0)≥0\),則\(g(x)\)的最小值為\(g(x_0)=0\),
因為由題意知\(0\)為\(g(x)\)的唯一零點,故\(x_0=0\),
即\(x_0=\log \dfrac{b}{a}\left(-\dfrac{\ln a}{\ln b}\right)=0\), \(-\dfrac{\ln a}{\ln b}=1\),
則\(\ln a+\ln b=0\),\(\ln ab=0\),\(ab=1\),
即\(ab\)值為\(1\). -
答案 (1)\([e,+∞)\);(2)\((0,e)\) .
解析 (1)\([e,+∞)\)(過程略);
(2)由題意得\(ae^x-x-2-\ln (x+2)+x+\ln a=0\)在\(x>-2\)上有兩個根,
即 \(e^{x+\ln a}+x+\ln a=\ln (x+2)+x+2=e^{\ln (x+2)}+\ln (x+2)\),(同構)
令\(h(x)=x+e^x\),則\(h(x)\)單調遞增,而\(h(x+\ln a)=h(\ln (x+2))\)
所以\(x+\ln a=\ln (x+2)\),問題轉化為\(\ln a=\ln (x+2)-x\)在\(x>-2\)時有\(2\)實根,
令\(t(x)=\ln (x+2)-x\),則 \(t^{\prime}(x)=-\dfrac{x+1}{x+2}\),
當\(-2<x<-1\)時,\(t'(x)>0\),\(t(x)\)單調遞增,
當\(x<-2\)或\(x>-1\)時,\(t'(x)<0\),\(t(x)\)單調遞減,
所以\(t(x)⩽t(-1)=1\),且在\((-2,-1)\),\((-1,+∞)\)上的值域都為\((-∞,-1]\),
綜上\(\ln a<1\),所以\(0<a<e\).
故\(a\)的取值範圍為\((0,e)\).
【B組---提高題】
1.己知函式\(f(x)=e^x-1-x+\dfrac{1}{2} ax^2\).
(1)當\(a≥0\)時,求\(f(x)\)的單調區間和極值;
(2)討論\(f(x)=e^x-1-x+\dfrac{1}{2} ax^2\)的零點的個數.
2.已知函式\(f(x)=(x-2)e^x+a(x-1)^2\)有兩個零點,求\(a\)的取值範圍.
參考答案
-
答案 (1)單調遞減區間為\((-∞,0)\),單調遞增區間為\((0,+∞)\);極小值為\(0\),無極大值;
(2) 當\(a≥0\)或\(a=-1\)時,\(f(x)\)有\(1\)個零點;
當\(a<0\)且\(a≠-1\)時,\(f(x)\)有\(2\)個零點.
解析 (1)\(f(x)=e^x-1-x+\dfrac{1}{2} ax^2\)的定義域為\((-∞,+∞)\),\(f'(x)=e^x-1+ax\),
\(\because a≥0\),\(\therefore f'' (x)=e^x+a>0\),
則\(f'(x)=e^x-1+ax\)在\((-∞,+∞)\)上單調遞增,
又\(f'(0)=0\),
所以當\(x\in (-∞,0)\)時,\(f'(x)<0\),當\(x\in (0,+∞)\)時,\(f'(x)>0\),
即\(f(x)\)的單調遞減區間為\((-∞,0)\),單調遞增區間為\((0,+∞)\);
\(f(x)\)的極小值為\(f(0)=0\),\(f(x)\)無極大值;
(2)當\(a≥0\)時,由(1)知\(f(x)≥f(0)=0\),
故\(f(x)\)僅有一個零點\(x=0\);
當\(a<0\)時,\(f'' (x)=e^x+a\),
令\(f'' (x)=0⇒x=\ln (-a)\);
令\(f'' (x)>0⇒x>\ln (-a)\),
所以\(f'(x)\)在\(x\in (\ln (-a),+∞)\)上單調遞增;
令\(f'' (x)<0⇒x<\ln (-a)\),
所以\(f'(x)\)在\(x\in (-∞,\ln (-a))\)上單調遞減,且\(f'(0)=0\),\(f(0)=0\),
所以\(f'(x)≥f'(\ln (-a))\),
最小值\(f'(\ln (-a))\)與\(0\)的比較等價於\(\ln (-a)\)與\(0\)的大小比較,
所以分三類進行討論:
①當\(-1<a<0\)時,即\(\ln (-a)<0\)時,
由\(f'(x)\)在\(x\in (-∞,\ln (-a))\)上單調遞減及在\(x\in (\ln (-a),+∞)\)上單調遞增,
且\(f'(0)=0\),\(x→-∞⇒f'(x)→+∞\)
由零點存在定理,得\(f'(x)\)在\(x\in (-∞,\ln (-a))\)上存在唯一零點,設為\(x_0\),所以
\(x\) | \((-∞,x_0 )\) | \(x_0\) | \((x_0,0)\) | \(0\) | \((0,+∞)\) |
---|---|---|---|---|---|
\(f'(x)\) | \(+\) | \(0\) | \(-\) | \(0\) | \(+\) |
\(f(x)\) | 遞增 | 極大值 | 遞減 | 極小值 \(f(0)=0\) | 遞增 |
又\(f(0)=0\)及\(x→-∞⇒f(x)→-∞\)由零點存在定理,
得\(f(x)\)在\(x\in (-∞,0)\)上存在唯一零點,設為\(x_1\),
綜上,當\(-1<a<0\)時,\(f(x)\)在\((-∞,+∞)\)上存在\(2\)個零點(一個為\(x=0\),一個為\(x_1\in (-∞,0)\));
②當\(a=-1\)時,即\(\ln (-a)=0\)時,
由\(f'(x)\)在\(x\in (-∞,0)\)上單調遞減及在\(x\in (0,+∞)\)上單調遞增,且\(f'(x)≥f'(0)=0\),
得\(f(x)\)在\((-∞,+∞)\)上單調遞增,
故\(f(x)\)在\((-∞,+∞)\)上只有一個零點\(x=0\);
③當\(a<-1\)時,
同理可得\(f(x)\)在\((-∞,+∞)\)上存在\(2\)個零點:一個為\(x=0\),一個為\(x_2\in (0,+∞)\),
綜上可得,當\(a≥0\)或\(a=-1\)時,\(f(x)\)有\(1\)個零點;
當\(a<0\)且\(a≠-1\)時,\(f(x)\)有\(2\)個零點.
-
答案 \((0,+∞)\)
解析 法一 分類討論
\(\because\)函式\(f(x)=(x-2)e^x+a(x-1)^2\),
\(\therefore f'(x)=(x-1)e^x+2a(x-1)=(x-1)(e^x+2a)\),
①若\(a=0\),那麼\(f(x)=0⇔(x-2)e^x=0⇔x=2\),
函式\(f(x)\)只有唯一的零點\(2\),不合題意;
②若\(a>0\),那麼\(e^x+2a>0\)恆成立,
當\(x<1\)時,\(f'(x)<0\),此時函式為減函式;
當\(x>1\)時,\(f'(x)>0\),此時函式為增函式;
此時當\(x=1\)時,函式\(f(x)\)取極小值\(-e\),
由\(f(2)=a>0\),可得:函式\(f(x)\)在\(x>1\)存在一個零點;
當\(x<1\)時,\(e^x<e\),\(x-2<x-1<0\),
\(\therefore f(x)=(x-2)e^x+a(x-1)^2>(x-2)e+a(x-1)^2=a(x-1)^2+e(x-1)-e\),
(使用放縮法)
方程\(a(x-1)^2+e(x-1)-e=0\)顯然由兩個不相等的實數根,
設為兩根為\(t_1\),\(t_2\),且\(t_1<1<t_2\),
則當\(x<t_1\)時, \(f(x)>a(x-1)^2+e(x-1)-e>0\),
(由\(x→-∞\)時,\(y=x-2→-∞\),\(y=e^x→0\),\(y=a(x-1)^2→+∞\),而二次函式比一次函式的增長速度快,\(f(x)→+∞\)可得函式\(f(x)\)在\(x<1\)存在一個零點,以上的證法更嚴謹但難度較大)
故函式\(f(x)\)在\(x<1\)存在一個零點;
即函式\(f(x)\)在\(R\)是存在兩個零點,滿足題意;
③若 \(-\dfrac{e}{2}<a<0\),則\(\ln (-2a)<\ln e=1\),
當\(x<\ln (-2a)\)或\(x>1\)時,\(f'(x)=(x-1)(e^x+2a)>0\)恆成立,
故\(f(x)\)單調遞增,
當\(\ln (-2a)<x<1\)時,\(f'(x)=(x-1)(e^x+2a)<0\)恆成立,
故\(f(x)\)單調遞減,
故當\(x= \ln (-2a)\)時,函式取極大值,
由\(f(\ln (-2a))=[\ln (-2a)-2](-2a)+a[\ln (-2a)-1]^2=a{[\ln (-2a)-2]^2+1}<0\)得:函式\(f(x)\)在\(R\)上至多存在一個零點,不合題意;
④若 \(a=-\dfrac{e}{2}\),則\(\ln (-2a)=1\),函式\(f(x)\)在\(R\)上單調遞增,
函式\(f(x)\)在\(R\)上至多存在一個零點,不合題意;
⑤若\(a<-\dfrac{e}{2}\),則\(\ln (-2a)>\ln e=1\),
當\(x<1\)或\(x>\ln (-2a)\)時,\(f'(x)=(x-1)(e^x+2a)>0\)恆成立,
故\(f(x)\)單調遞增,
當\(1<x<\ln (-2a)\)時,\(f'(x)=(x-1)(e^x+2a)<0\)恆成立,
故\(f(x)\)單調遞減,
故當\(x=1\)時,函式取極大值,
由\(f(1)=-e<0\)得:函式\(f(x)\)在\(R\)上至多存在一個零點,不合題意;
綜上所述,\(a\)的取值範圍為\((0,+∞)\).
法二 分類引數
解:顯然\(x=1\)不是函式\(f(x)\)的零點,
當\(x≠1\)時,方程\(f(x)=0\)等價於 \(-a=\dfrac{x-2}{(x-1)^2} \cdot e^x\),
設 \(g(x)=\dfrac{x-2}{(x-1)^2} \cdot e^x\),求導 \(g^{\prime}(x)=e^x \cdot \dfrac{x^2-4 x+5}{(x-1)^3}\),
當\(x<1\)時,\(g'(x)<0\);當\(x>1\)時,\(g'(x)>0\);
\(\therefore\)函式\(g(x)\)在\((-∞,1)\)上單調遞減,在\((1,+∞)\)上單調遞增,
且\(x→-∞\)時,\(g(x)→0\);\(x→1\)時,\(g(x)→-∞\);\(x→+∞\)時,\(g(x)→+∞\);
\(\therefore\)函式\(g(x)\)在\((-∞,1)\)上的值域為\((-∞,0)\),在\((1,+∞)\)上的值域為\((-∞,+∞)\),
\(\therefore\)當\(-a<0\)時,函式\(f(x)\)有兩個零點,
故所求\(a\)取值範圍為\((0,+∞)\).
【C組---拓展題】
1.已知函式\(f(x)=\ln x\).
(1)若\(af(x)⩽e^{x-1}-1\)恆成立,求實數\(a\)的值;
(2)若關於\(x\)的方程\(f\left(x^2\right)-x+\dfrac{m}{x}-\ln m=0\)有四個不同的實數根,則實數\(m\)的取值範圍.
2.已知函式\(f(x)=\ln x-a^2 x^2+ax\).
(1)求函式\(f(x)\)在定義域內的最值.
(2)當\(a>0\)時,若\(y=f'(x)\)有兩個不同的零點\(x_1\),\(x_2\),求證:\(a(x_1+x_2 )>2\).
參考答案
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答案 (1) \(a=1\);(2) \((0,1)\) .
解析 (1)\(a=1\).(過程略)
(2)解法一:由題意知,\(m>0\), \(h(x)=\ln x^2-x+\dfrac{m}{x}-\ln m\),
\(h^{\prime}(x)=\dfrac{2}{x}-1-\dfrac{m}{x^2}=\dfrac{-x^2+2 x-m}{x^2}\),
當\(x<0\)時,\(h'(x)<0\),\(h(x)\)在區間\((-∞,0)\)單調遞減,
又 \(h(-\sqrt{m})=0\),故\(h(x)\)在區間\((-∞,0)\)有唯一實根,
若\(m⩾1\),\(-x^2+2x-m=-(x-1)^2+1-m⩽0\),
當\(x>0\)時,\(h'(x)≤0\),\(h(x)\)在區間\((0,+∞)\)單調遞減,
故\(h(x)\)在區間\((0,+∞)\)至多有一個實根,不符合題意,
若\(0<m<1\),令\(x_1,x_2 (x_1<x_2 )\)是方程\(-x^2+2x-m=0\)的兩不同實根,
易得\(0<x_1<1<x_2\),
故\(h(x)\)在區間\((0,x_1 )\),\((x_2,+∞)\)上單調遞減,在區間\((x_1,x_2 )\)上單調遞增.
\(h\left(x_1\right)=\ln x_1^2-x_1+\dfrac{m}{x_1}-\ln m=\ln x_1^2-x_1+\dfrac{-x_1^2+2 x_1}{x_1}-\ln \left(-x_1^2+2 x_1\right)\)
\(=-2x_1+2+\ln x_1-\ln (2-x_1 )\),
設\(φ(x)=-2x+2+\ln x-\ln (2-x)(0<x<1)\),
則 \(\varphi^{\prime}(x)=-2+\dfrac{1}{x}+\dfrac{1}{2-x}=\dfrac{2(x-1)^2}{x(2-x)}>0\),
\(φ(x)<φ(1)=0\),\(h(x_1 )<0\),
同理可證\(h(x_2 )>0\),
取 \(x_3=\left(1+\sqrt{2+\dfrac{1}{m}}\right)^2>x_2=1+\sqrt{1-m}\), \(h\left(x_3\right)<2 \sqrt{x_3}-x_3+1+\dfrac{1}{m}=0\).
取 \(x_4=\min \left\{\ln \dfrac{1}{m}, \dfrac{m^2}{4}\right\}\), \(x_4 \leqslant \dfrac{m^2}{4}<\dfrac{m}{2}<x_1=1-\sqrt{1-m}\),
\(h\left(x_4\right)>2 \sqrt{x_4}-\dfrac{2}{\sqrt{x_4}}+\dfrac{m}{x_4}+\left(\ln \dfrac{1}{m}-x_4\right)=2 \sqrt{x_4}+\dfrac{m-2 \sqrt{x_4}}{x_4}+\left(\ln \dfrac{1}{m}-x_4\right)>0\).
故\(h(x)\)在\((x_4,x_1 )\),\((x_1,x_2 )\),\((x_2,x_3 )\)各存在一個零點,
實數\(m\)的取值範圍是\((0,1)\).
解法二:由題意知,\(m>0\), \(h(x)=\ln x^2-x+\dfrac{m}{x}-\ln m\),
\(h^{\prime}(x)=\dfrac{2}{x}-1-\dfrac{m}{x^2}=\dfrac{-x^2+2 x-m}{x^2}\),
當\(x<0\)時,\(h'(x)<0\),\(h(x)\)在區間\((-∞,0)\)單調遞減,
又 \(h(-\sqrt{m})=0\),故\(h(x)\)在區間\((-∞,0)\)有唯一實根,
因此當\(x>0\)時,\(\ln x-x=\ln \dfrac{m}{x}-\dfrac{m}{x}\)有三個實根,
當\(x=\dfrac{m}{x}\), \(x=\sqrt{m}\)是方程的一個實根,
若\(x \neq \dfrac{m}{x}\), \(\ln x-\ln \dfrac{m}{x}=x-\dfrac{m}{x}\), \(\dfrac{1}{\sqrt{m}}>\dfrac{\ln x-\ln \dfrac{m}{x}}{x \dfrac{m}{x}}=1\)(對數均值不等式),
所以\(0<m<1\).
故\(m\)的取值範圍為\((0,1)\). -
答案 (1) 最大值\(-\ln a\),無最小值;(2)略 .
解析 (1)函式\(f(x)=\ln x-a^2 x^2+ax\)的定義域為\((0,+∞)\),
\(f^{\prime}(x)=\dfrac{1}{x}-2 a^2 x+a=-\dfrac{(2 a x+1)(a x-1)}{x}\),
當\(a=0\)時,\(f(x)=\ln x\),
此時函式\(f(x)\)為增函式,無最值,
當\(a≠0\)時,令\(f'(x)=0\),得\(x=-\dfrac{1}{2a}\)或\(x=\dfrac{1}{a}\),
①若\(a<0\),則\(-\dfrac{1}{2a}>0\),\(\dfrac{1}{a} <0\),
由\(f'(x)>0\),得\(0<x<-\dfrac{1}{2a}\),\(f(x)\)單調遞增,
由\(f'(x)<0\),得\(x>-\dfrac{1}{2a}\),\(f(x)\)單調遞減,
所以函式\(f(x)\)在定義域內有最大值\(f\left(-\dfrac{1}{2 a}\right)=\ln \left(-\dfrac{1}{2 a}\right)-a^2\left(-\dfrac{1}{2 a}\right)^2+a\left(\dfrac{1}{a}\right)=\ln \left(-\dfrac{1}{2 a}\right)-\dfrac{3}{4}\),無最小值.
②若\(a>0\),則\(-\dfrac{1}{2a}<0\),\(\dfrac{1}{a} >0\),
由\(f'(x)>0\),得\(0<x<\dfrac{1}{a}\) ,\(f(x)\)單調遞增,
由\(f'(x)<0\),得\(x>\dfrac{1}{a}\),\(f(x)\)單調遞減,
所以\(f(x)\)定義域內有最大值\(f\left(\dfrac{1}{a}\right)=\ln \left(\dfrac{1}{a}\right)-a^2\left(\dfrac{1}{a}\right)^2+a\left(-\dfrac{1}{2 a}\right)=-\ln a\),無最小值.
(2)證明:由(1)知,當\(a>0\)時,函式\(f(x)\)在定義域內的最大值為\(f\left(\dfrac{1}{a}\right)=-\ln a\),
因為\(y=f(x)\)有兩個不同的零點\(x_1\),\(x_2\),
所以\(-\ln a>0\),解得\(-\ln a>0\), 解得\(0<a<1\),
不妨設\(0<x_1<\dfrac{1}{a} <x_2\),
由題意知\(f(x_1 )=\ln x_1-a^2 x_1^2+ax_1=0\),\(f(x_2 )=\ln x_2-a^2 x_2^2+ax_2=0\),
所以\(\ln x_2-\ln x_1=a^2 (x_2^2-x_1^2 )-ax_2+ax_1\),
即\(\ln \dfrac{\ln x_2-\ln x_1}{x_2-x_1}=a\left[a\left(x_2+x_1\right)-1\right]\),
即\(\dfrac{\ln \dfrac{x_2}{x_1}}{\dfrac{x_2}{x_1}-1}=a\left[a\left(x_1+x_2\right)-1\right] x_1\),
設 \(h(t)=\ln t-\dfrac{2(t-1)}{t+1}(t>1)\),
則 \(h^{\prime}(t)=\dfrac{1}{t}-\dfrac{4}{(t+1)^2}=\dfrac{(t-1)^2}{t(t+1)^2}(t>1)\),
所以當\(t>1\)時,\(h'(t)>0\),\(h(t)\)單調遞增,
所以\(h(t)>h(1)=0\),
即\(\ln t-\dfrac{2(t-1)}{t+1}>0\),所以 \(\dfrac{\ln t}{t-1}>\dfrac{2}{t+1}\),
令\(t=\dfrac{x_2}{x_1}\) ,
則上式可化為 \(\dfrac{\ln \dfrac{x_2}{x_1}}{\dfrac{x_2}{x_1}-1}>\dfrac{2}{\dfrac{x_2}{x_1}+1}\),
所以 \(a\left[a\left(x_1+x_2\right)-1\right] x_1>\dfrac{2}{\dfrac{x_2}{x_1}+1}\),
所以 \(a\left[a\left(x_1+x_2\right)-1\right] x_1>\dfrac{2}{x_1+x_2}\),
即\(a^2 (x_1+x_2 )^2-a(x_1+x_2 )-2>0\),
所以\([a(x_1+x_2 )-2][a(x_1+x_2 )+1]>0\),
又因為\(a(x_1+x_2 )+1>0\)恆成立,
所以\(a(x_1+x_2 )>2\).