5.3.2(1) 導數與函式的極值
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基礎知識
極值的概念
若在點\(x=a\)附近的左側\(f'(x)<0\),右側\(f'(x)>0\), 則\(a\)稱為函式\(y=f(x)\)的極小值點,\(f(a)\)稱為函式\(y=f(x)\)的極小值;
若在點\(x=b\)附近的左側\(f'(x)>0\)
極小值點、極大值點統稱為極值點,極大值和極小值統稱為極值.
解釋
① 把函式圖象看成一座“山脈”,極大值就是“山峰”,極小值就是“山谷”, 如下圖;
② 極值是“函式值\(y\)”,極值點是“自變數\(x\)值”,如下圖有極大值\(f(-1)\)和\(f(1)\),極小值\(f(-2)\)和\(f(2)\),極大值點\(-1\)和\(1\),極小值點\(-2\)和\(2\).
③ 極值反映了函式在某一點附近的大小情況,刻畫了函式的區域性性質;
④ 對於極值還有特別強調一下,看例題:
【例】
A. 必有\(f'(x_0 )=0\) \(\qquad \qquad \qquad \qquad\qquad \qquad\) B.\(f'(x_0 )\)不存在
C. \(f'(x_0 )=0\)或\(f'(x_0 )\)不存在 \(\qquad \qquad \qquad \qquad\) D. \(f'(x_0 )\)存在但可能不為\(0\)
解析 函式\(f(x)=x^3\),
\(f'(x)=3x^2\),\(f'(0)=0\),
但\(x<0\)時,\(f'(x)>0\)
故根據極值的定義,\(0\)不是函式\(f(x)=x^3\)的極值點,這個從函式圖象也很容易知道.
又如函式\(g(x)=|x|\),
當\(x<0\)時,\(g'(x)=-1<0\); 當\(x>0\)時,\(g'(x)=1>0\);
所以\(g(x)\)在\(x=0\)處取到極值,但在導數不存在;故選\(C\).
總結
① 若\(f(x)\)可導,且\(x_0\)是\(y=f(x)\)的極值,則\(x_0\)是\(f'(x)=0\)的解;
② 若\(x_0\)是\(f'(x)=0\)的解,\(x_0\) 不一定是\(y=f(x)\)的極值點;
③ 定義很重要.
【例】判斷\(x=0\)是否\(f(x)= \dfrac{1}{3} x^3-x^2\)的極大值點.
解 \(\because f(x)= \dfrac{1}{3} x^3-x^2\),\(\therefore f'(x)=x^2-2x\),
當\(x>0\)時,\(f'(x)<0\);當\(x<0\)時,\(f'(x)>0\);
所以\(x=0\)是\(f(x)= \dfrac{1}{3} x^3-x^2\)的極大值點.
求函式的極值的方法
解方程\(f'(x)=0\) ,當\(f'(x_0)=0\)時:
(1) 如果在\(x_0\)附近的左側\(f'(x)>0\),右側\(f'(x)<0\),那麼\(f(x_0)\)是極大值;
(2) 如果在\(x_0\)附近的左側\(f'(x)<0\),右側 \(f'(x)>0\),那麼\(f(x_0)\)是極小值.
基本方法
【題型1】 極值的概念
【典題1】(多選)設函式\(f(x)\)的定義域為\(R\),\(x_0 (x_0≠0)\)是\(f(x)\)的極大值點,以下結論錯誤的是( )
A.\(∀x∈R\),\(f(x)≤f(x_0)\) \(\qquad \qquad \qquad \qquad \qquad\) B.\(-x_0\)是\(f(-x)\)的極小值點
C.\(-x_0\)是\(-f(x)\)的極小值點 \(\qquad \qquad \qquad \qquad\) D.\(-x_0\)是\(-f(-x)\)的極小值點
解析 \(A\).\(∀x∈R\),\(f(x)≤f(x_0)\),錯誤.\(x_0 (x_0≠0)\)是\(f(x)\))的極大值點,並不是最大值點;
\(B\).\(-x_0\)是\(f(-x)\)的極小值點,錯誤.\(f(-x)\)相當於\(f(x)\)關於\(y\)軸的對稱圖象,故\(-x_0\)應是\(f(-x)\)的極大值點;
\(C\).\(-x_0\)是\(-f(x)\)的極小值點,錯誤.\(-f(x)\)相當於\(f(x)\)關於\(x\)軸的對稱圖象,故\(x_0\)應是\(-f(x)\)的極小值點.跟\(-x_0\)沒有關係;
\(D\).\(-x_0\)是\(-f(-x)\)的極小值點,正確.\(-f(-x)\)相當於\(f(x)\)先關於\(y\)軸的對稱,再關於\(x\)軸的對稱圖象.故\(D\)正確.
故選:\(ABC\).
點撥 結合圖象分析.
【典題2】若函式 \(f(x)=x^3-\left(\dfrac{a}{2}+3\right) x^2+2 a x+3\)在\(x=2\)處取得極小值,則實數\(a\)的取值範圍是( )
A.\((-∞,-6)\) \(\qquad \qquad \qquad\) B.\((-∞,6)\) \(\qquad \qquad \qquad\) C.\((6,+∞)\) \(\qquad \qquad \qquad\) D.\((-6,+∞)\)
解析 \(f(x)=x^3-\left(\dfrac{a}{2}+3\right) x^2+2 a x+3\),
則\(f'(x)=3x^2-(a+6)x+2a\),
由題意得:\(f'(2)=0\),即\(12-2a-12+2a=0\),\(f'(2)\)恆為\(0\),
\(\because f(2)\)是極小值,\(\therefore x<2\)時,函式單調遞減,\(x>2\)時,函式單調遞增,
結合二次函式的性質\(f'(x)\)的對稱軸在\(x=2\)的左側,
即\(\dfrac{a+6}{6}<2\),故\(a<6\),
又\(△=(a+6)^2-24a=(a-6)^2>0\),故\(a<6\),
故選:\(B\).
點撥 本題依題意得\(f'(2)=0\),由於等式恆成立,求不出\(a\),則利用極值的概念求出\(a\)的取值範圍.
【鞏固練習】
1.已知函式\(y=f(x)\)的導函式的圖象如圖所示,則下列結論正確的是( )
A.\(-4\)是函式\(f(x)\)的極小值點 \(\qquad \qquad \qquad \qquad\) B.\(-1\)是函式\(f(x)\)的極小值點
C.函式\(f(x)\)在區間\((-4,1)\)上單調遞減 \(\qquad \qquad\) D.函式\(f(x)\)在區間\((-4,-1)\)上先增後減
2.函式\(f(x)=4x^3-ax^2-2bx+2\)在\(x=1\)處有極值,則\(a+b\)的值等於( )
A.\(9\) \(\qquad \qquad \qquad \qquad\) B.\(6\) \(\qquad \qquad \qquad \qquad\) C.\(3\) \(\qquad \qquad \qquad \qquad\) D.\(2\)
3.已知函式\(f(x)=x^3+2ax^2+bx+a^2\)在\(x=1\)處的極小值為\(6\),則數對\((a,b)\)為( )
A.\((-2,5)\) \(\qquad \qquad\) B.\((-19,4)\) \(\qquad \qquad\) C.\((4,-19)\) \(\qquad \qquad\) D.\((-2,5)\)或\((4,-19)\)
4.(多選)已知函式\(f(x)= \dfrac{1}{3} x^3+x^2-2ax+1\),當實數\(a\)為下列( )的值時,函式\(f(x)\)在\((1,2)\)上有極值.
A.\(1\) \(\qquad \qquad \qquad \qquad\) B.\(2\)\(\qquad \qquad \qquad \qquad\) C.\(3\) \(\qquad \qquad \qquad \qquad\) D.\(4\)
參考答案
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答案 \(A\)
解析 結合導函式的圖象,可知\(f(x)\)在\((-∞,-4)\)單調遞減,在\((-4,+∞)\)單調遞增,
所以\(-4\)是函式\(f(x)\)的極小值點,故\(A\)正確;
\(-1\)不是\(f(x)\)的極值點,故\(B\)錯誤;
函式\(f(x)\)在區間\((-4,1)\)上單調遞增,故\(C\)錯誤;
函式\(f(x)\)在區間\((-4,-1)\)上單調遞增,故\(D\)錯誤;
故選:\(A\). -
答案 \(B\)
解析 函式\(f(x)=4x^3-ax^2-2bx+2\)在\(x=1\)處有極值,可知\(f'(1)=0\),
而\(f'(x)=12x^2-2ax-2b\),故\(12-2a-2b=0\) ,故\(a+b=6\),
故選:\(B\). -
答案 \(C\)
解析 由\(f(x)=x^3+2ax^2+bx+a^2\),得\(f'(x)=3x^2+4ax+b\),
\(\because f(x)\)在\(x=1\)處的極小值為\(6\),\(\therefore f'(1)=0\)且\(f(1)=6\),
\(\therefore 3+4a+b=0\)且\(1+2a+b+a^2=6\),
\(\therefore a=-2\),\(b=5\)或\(a=4\),\(b=-19\),
經檢驗當\(a=-2\),\(b=5\)時,\(f(x)\)在\(x=1\)處取不到極小值\(6\),
\(\therefore a=4\),\(b=-19\), \(\therefore\)數對\((a,b)\)為\((4,-19)\).
故選:\(C\). -
答案 \(BC\)
解析 \(f'(x)=x^2+2x-2a\),
因為函式\(f(x)\)在\((1,2)\)上有極值,所以\(f'(x)=0\)在\((1,2)\)上有根,
所以\(y=f'(x)\)在\((1,2)\)上有變號零點,
又因為\(f'(x)=x^2+2x-2a\),在\((1,2)\)上單調遞增,
所以\(\left\{\begin{array}{l} f^{\prime}(1)=3-2 a<0 \\ f^{\prime}(2)=8-2 a>0 \end{array}\right.\),解得\(\dfrac{3}{2}<a<4\),
故選:\(BC\).
【題型2】 不含參函式的極值
【典題1】 已知曲線\(f(x)=ax^3-bx^2+2\)在點\((1,f(1))\)處的切線方程為\(y=1\).
(1)求\(a\)、\(b\)的值;\(\qquad \qquad\) (2)求\(f(x)\)的極值.
解析 (1)由函式的解析式可得\(f'(x)=3ax^2-2bx\),
由切線方程可知切點座標為\((1,1)\),切線的斜率為\(0\),
從而有 \(\left\{\begin{array}{l}
f(1)=a-b+2=0 \\
f^{\prime}(1)=3 a-2 b=0
\end{array}\right.\),求解方程組可得 \(\left\{\begin{array}{l}
a=2 \\
b=3
\end{array}\right.\),
故\(a=2\),\(b=3\).
(2)由題意可得\(f(x)=2x^3-3x^2+2\),\(f'(x)=6x^2-6x\),
當\(x∈(-∞,0)\)時,\(f'(x)>0\),\(f(x)\)單調遞增,
當\(x∈(0,1)\)時,\(f'(x)<0\),\(f(x)\)單調遞減,
當\(x∈(1,+∞)\)時,\(f'(x)>0\),\(f(x)\)單調遞增,
故函式的極大值為\(f(0)=2\),函式的極小值為\(f(1)=1\).
點撥 求函式極值先求函式單調性,解題步驟與思路差不多;藉助導函式的“穿線圖”更好得到結論.
【鞏固練習】
1.下列函式中,不存在極值點的是( )
A.\(y=x+ \dfrac{1}{x}\) \(\qquad \qquad\) B. \(y=2^{|x|}\) \(\qquad \qquad\) C.\(y=x⋅\ln x\) \(\qquad \qquad\) D.\(y=-2x^3-x\)
2.函式\(y=1+3x-x^3\)有( )
A.極小值\(-1\),極大值\(1\) \(\qquad \qquad \qquad \qquad\) B.極小值\(-1\),極大值\(3\)
C.極小值\(-2\),極大值\(2\) \(\qquad \qquad \qquad \qquad\) D.極小值\(2\),極大值\(3\)
3.函式\(f(x)=\cos 2x+\sin x\),\(x \in\left(0, \dfrac{\pi}{2}\right)\)的極值點為\(x_0\),則\(\tan x_0\)的值為( )
A.\(\dfrac{\sqrt{15}}{15}\) \(\qquad \qquad \qquad \qquad\) B.\(\sqrt{15}\) \(\qquad \qquad \qquad \qquad\) C.\(\sqrt{3}\) \(\qquad \qquad \qquad \qquad\) D. \(\dfrac{\sqrt{3}}{3}\)
4.已知函式\(f(x)=a\ln x- \dfrac{1}{2} x^2\),\((a∈R)\)在\(x=1\)處的切線與直線\(y=x+1\)平行.
(1)求實數a的值;\(\qquad \qquad\) (2)求函式\(f(x)\)的極值.
參考答案
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答案 \(D\)
解析 \(y=x+ \dfrac{1}{x}\),\(x=-1\)時,函式取得極大值,\(x=1\)時,函式取得極小值,所以\(A\)不正確;
\(y=2^{|x|}\)是偶函式,\(x=0\)時,函式取得極小值,所以\(B\)不正確;
\(y=x⋅\ln x\)可得\(y'=\ln x+1\), \(x>\dfrac{1}{e}\)時,\(y'>0\),函式是增函式,
\(x \in\left(0, \dfrac{1}{e}\right)\)時,\(y'<0\),函式是減函式,\(x=\dfrac{1}{e}\)時,函式取得極小值,所以\(C\)不正確.
\(y=-2x^3-x\),可得\(y'=-6x^2-1<0\)恆成立,函式是減函式,
所以函式沒有極小值,所以\(D\)正確.
故選:\(D\). -
答案 \(B\)
解析 \(y'=3-3x^2\),
令\(y'=0\),解得\(x=±1\),
\(x∈(-∞,-1)\),\((1,+∞)\)時,\(y'<0\),\(x∈(-1,1)\)時,\(y'>0\),
\(\therefore\)函式\(y=1+3x-x^3\)有在\((-∞,-1)\),\((1,+∞)\)上遞減,在\((-1,1)\)遞增,
\(\therefore x=1\),函式取得極大值\(1+3×1-1^3=3\),
\(x=-1\)時,函式取得極小值\(1+3×(-1)-(-1)^3=-1\),
故選:\(B\). -
答案 \(A\)
解析 函式\(f(x)=\cos 2x+\sin x\),\(x \in\left(0, \dfrac{\pi}{2}\right)\),
所以 \(f^{\prime}(x)=-2 \sin 2 x+\cos x=\cos x(1-4 \sin x)\),
令\(f'(x)=0\),\(\cos x(1-4\sin x)=0\),
因為\(x \in\left(0, \dfrac{\pi}{2}\right)\),所以\(\cos x≠0\),
所以\(1-4\sin x=0\),即\(\sin x= \dfrac{1}{4}\),
因為函式\(f(x)=\cos 2x+\sin x\),\(x \in\left(0, \dfrac{\pi}{2}\right)\)的極值點為\(x_0\),
所以\(\sin x_0= \dfrac{1}{4}\), \(\cos x_0=\dfrac{\sqrt{15}}{4}\), \(\therefore \tan x_0=\dfrac{\sqrt{15}}{15}\),
故選:\(A\). -
答案 (1) \(a=2\);(2) 極大值為\(\ln 2-1\),無極小值.
解析 (1)由題意可知,\(f'(1)=1\),
\(\because f^{\prime}(x)=\dfrac{a}{x}-x\),\(\therefore f'(1)=a-1=1\),\(\therefore a=2\).
(2) \(\because f^{\prime}(x)=\dfrac{2}{x}-x=\dfrac{2-x^2}{x}\),
\(\therefore f'(x)>0⇒0<x<\sqrt{2}\),\(f'(x)<0⇒x>\sqrt{2}\),
即函式\(f(x)\)在\((0,\sqrt{2})\)上單調遞增,在\((\sqrt{2},+∞)\)上單調遞減,
故函式\(f(x)\)的極大值為\(f(\sqrt{2})=2\ln \sqrt{2}-1=\ln 2-1\),無極小值.
【題型3】含參函式的極值
【典題1】討論函式\(f(x)=e^x-2ax-a\)的極值.
解析 \(f'(x)=e^x-2a\),
當\(a≤0\)時,\(f'(x)= e^x-2a>0\),\(f(x)\)在\(R\)上為單調增函式,無極值,
當\(a>0\)時,
由\(f'(x)= e^x-2a>0\),\(x>\ln (2a)\),\(f(x)\)在\((\ln (2a),+∞)\)上為單調增函式,
由\(f'(x)= e^x-2a<0\),\(x<\ln (2a)\),\(f(x)\)在\((-∞,\ln (2a))\)上為單調減函式,
所以 \(f_{\text {極小值 }}=f(\ln (2 a))=a-2 a \ln (2 a)\),無極大值.
綜上所述:當\(a≤0\)時,無極值,
當\(a>0\)時, \(f_{\text {極小值 }}=a-2 a \ln (2 a)\),無極大值.
【鞏固練習】
1.討論\(f(x)=x^3-ax^2+2\)的極值.
2.討論\(f(x)=\ln x+ \dfrac{1}{2} x^2+ax(a∈R)\)的極值點的個數.
參考答案
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答案 當\(a≤0\)時,\(f(x)\)無極值;當\(a>0\)時,極小值\(-\dfrac{4 a^3}{27}+2\),無極大值.
解析 定義域\(x∈(0,+∞)\),\(f'(x)=3x^2-2ax=x(3x-2a)\),
當\(a≤0\)時,\(f'(x)≥0\),\(f(x)\)在\((0,+∞)\)上單增,無極值,
當\(a>0\)時, \(f^{\prime}(x)>0 \Rightarrow x>\dfrac{2 a}{3}\),
\(\therefore f(x)\)單減區間是\(\left(0, \dfrac{2 a}{3}\right)\),單增區間是\(\left(\dfrac{2 a}{3},+\infty\right)\),
所以 \(f(x)_{\text {極小 }}=f\left(\dfrac{2 a}{3}\right)=-\dfrac{4 a^3}{27}+2\),無極大值.
綜上所述,當\(a≤0\)時,\(f(x)\)無極值;當\(a>0\)時,極小值\(-\dfrac{4 a^3}{27}+2\),無極大值. -
答案 當\(a≥-2\)時,函式\(f(x)\)無極值點;當\(a<-2\)時,\(f(x)\)有兩個極值點.
解析 由題意得: \(f^{\prime}(x)=\dfrac{1}{x}+x+a=\dfrac{x^2+a x+1}{x}(x>0)\),
令\(f'(x)=0\),即\(x^2+ax+1=0\),\(△=a^2-4\),
①當\(△=a^2-4≤0\),即\(-2≤a≤2\)時,
\(x^2+ax+1≥0\)對任意的\(x>0\)恆成立,
即\(f^{\prime}(x)=\dfrac{x^2+a x+1}{x} \geq 0\)對任意\(x>0\)恆成立,此時\(f(x)\)沒有極值點;
②當\(△=a^2-4>0\),即\(a<-2\)或\(a>2\)時,
若\(a<-2\),設方程的兩根不同實根\(x_1\),\(x_2\),
不妨設\(x_1<x_2\),
則\(x_1+x_2=-a\),\(x_1 x_2=1>0\),故\(x_2>x_1>0\),
當\(0<x<x_1\) 或\(x>x_2\)時,\(f'(x)>0\),
當\(x_1<x<x_2\)時,\(f'(x)<0\),
故\(x_1\),\(x_2\)是函式\(f(x)\)的兩個極值點.
若\(a>2\),設方程\(x^2+ax+1=0\)的兩個不同的實根\(x_1\),\(x_2\),
則\(x_1+x_2=-a<0\),\(x_1 x_2=1\),故\(x_1<0\),\(x_2<0\),
所以當\(x>0\)時,\(f'(x)>0\),故函式\(f(x)\)沒有極值點,
綜上當\(a≥-2\)時,函式\(f(x)\)無極值點;當\(a<-2\)時,\(f(x)\)有兩個極值點.
分層練習
【A組---基礎題】
1.已知函式\(f(x)\)的導函式為\(f'(x)\),函式\(g(x)=(x-1)f'(x)\)的圖象如圖所示,則下列結論正確的是( )
A.\(f(x)\)在\((-∞,-2)\),\((1,2)\)上為減函式 \(\qquad \qquad \qquad \qquad\) B.\(f(x)\)在\((-2,1)\),\((2,+∞)\)上為增函式
C.\(f(x)\)的極小值為\(f(-2)\),極大值為\(f(2)\) \(\qquad \qquad \qquad \qquad\) D.\(f(x)\)的極大值為\(f(-2)\),極小值為\(f(2)\)
2.函式\(f(x)= \dfrac{1}{2} x^2+\ln x-2x\)的極值點的個數是( )
A.\(0\) \(\qquad \qquad \qquad \qquad\) B.\(1\) \(\qquad \qquad \qquad \qquad\) C.\(2\) \(\qquad \qquad \qquad \qquad\) D.無數個
3.若函式\(f(x)= \dfrac{1}{2} x^2-x+a\ln x\)有兩個不同的極值點,則實數\(a\)的取值範圍是( )
A.\(a> \dfrac{1}{4}\) \(\qquad \qquad \qquad\) B.\(- \dfrac{1}{4}<a<0\) \(\qquad \qquad \qquad\) C.\(a< \dfrac{1}{4}\) \(\qquad \qquad \qquad\) D.\(0<a< \dfrac{1}{4}\)
4.已知函式\(f(x)=x\ln x+x^2\),且\(x_0\)是函式\(f(x)\)的極值點.給出以下幾個問題:
①\(x_0> \dfrac{1}{e}\);②\(0<x_0< \dfrac{1}{e}\);③\(f(x_0 )+x_0<0\);④\(f(x_0 )+x_0>0\)
其中正確的命題是( )
A.①③ \(\qquad \qquad \qquad \qquad\) B.①④ \(\qquad \qquad \qquad \qquad\) C.②③ \(\qquad \qquad \qquad \qquad\) D.②④
5.已知函式\(f(x)=x^3+3ax^2+1(a<0)\),\(f(x)\)極小值點為\(x_0\),若\(f(x_1 )=f(x_0 )\)且\(x_1≠x_0\),則\(x_1⋅f(x_1+x_0 )\)的最小值為( )
A.\(- \dfrac{3}{4}\) \(\qquad \qquad \qquad \qquad\) B. \(- \dfrac{3}{8}\) \(\qquad \qquad \qquad \qquad\) C. \(\dfrac{3}{8}\) \(\qquad \qquad \qquad \qquad\) D. \(\dfrac{3}{4}\)
6.若函式\(f(x)=m\cdot e^x-x^2+2x(m<0)\)在\((0,1)\)上有極值點,則\(m\)的取值範圍為\(\underline{\quad \quad}\).
7.設\(x_1\)和\(x_2\)是函式\(f(x)=x^3+2ax^2+x+1\)的兩個極值點.若\(x_2-x_1=2\),則\(a^2=\)\(\underline{\quad \quad}\).
8.設函式\(f(x)=2x^3+3x^2+ax+b\),曲線\(y=f(x)\)在點\((0,f(0))\)處的切線方程為\(y=-12x+1\).
(1)求\(f(x)\)的解析式;\(\qquad \qquad\) (2)求\(f(x)\)的極值.
9.設\(a\)為實數,函式\(f(x)= \dfrac{1}{3} x^3- \dfrac{1}{2}(a-1)x^2-ax(x∈R)\).
(1)當\(a=1\)時,求\(f(x)\)的單調區間;
(2)求\(f(x)\)在\(R\)上的極大值與極小值.
參考答案
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答案 \(D\)
解析 當\(x∈(-∞,-2)\)時,\(x-1<0\),由圖象可得\(g(x)=(x-1)f'(x)<0\),
則\(f'(x)>0\),\(f(x)\)為增函式;
當\(x∈(-2,1)\)時,\(x-1<0\),由圖象可得\(g(x)=(x-1)f'(x)>0\),
則\(f'(x)<0\),\(f(x)\)為減函式;
當\(x∈(1,2)\)時,\(x-1>0\),由圖象可得\(g(x)=(x-1)f'(x)<0\),
則\(f'(x)<0\),\(f(x)\)為減函式;
當\(x∈(2,+∞)\)時,\(x-1>0\),由圖象可得\(g(x)=(x-1)f'(x)>0\),
則\(f'(x)>0\),\(f(x)\)為增函式,
所以\(f(x)\)的極大值為\(f(-2)\),極小值為\(f(2)\),
結合選項可知,只有選項\(D\)正確.
故選:\(D\). -
答案 \(A\)
解析 \(f^{\prime}(x)=x+\dfrac{1}{x}-2=\dfrac{(x-1)^2}{x} \geqslant 0\),
故原函式單調遞增,\(f(x)\)無極值點.
故選:\(A\). -
答案 \(D\)
解析 因為\(f(x)= \dfrac{1}{2} x^2-x+a\ln x\)有兩個不同的極值點,
所以\(f^{\prime}(x)=x-1+\dfrac{a}{x}=\dfrac{x^2-x+a}{x}=0\)在\((0,+∞)\)有\(2\)個不同的零點,
所以\(x^2-x+a=0\)在\((0,+∞)\)有\(2\)個不同的零點,
所以\(\left\{\begin{array}{l} \triangle=1-4 a>0 \\ a>0 \end{array}\right.\),解可得,\(0<a< \dfrac{1}{4}\).
故選:\(D\). -
答案 \(C\)
解析\(f(x)\)的定義域為\(x>0\),\(f'(x)=\ln x+2x+1\),
所以有$f'(x_0 )=\ln x_0+2x_0+1=$0,
所以有\(2x_0=-(\ln x_0+1)>0\),即\(\ln x_0<-1\),
即 \(\ln x_0<\ln e^{-1}\),所以有\(0<x_0< \dfrac{1}{e}\);故①錯誤,②正確;
\(f(x_0 )+x_0=x_0 \ln x_0+x_0^2+x_0=x_0 (\ln x_0+x_0+1)\),
因為\(2x_0=-(\ln x_0+1)\),
所以有\(f(x_0 )+x_0=x_0 \ln x_0+x_0^2+x_0=x_0 (\ln x_0+x_0+1)=-x_0^2<0\),
故③正確,④錯誤.
故選:\(C\). -
答案 \(B\)
解析 \(f'(x)=3x^2+6ax=3x(x+2a)\),
\(\because a<0\),
當\(x>-2a\)或\(x<0\)時,\(f'(x)>0\),函式單調遞增,
當\(0<x<-2a\)時,\(f'(x)<0\),函式單調遞減,
故極小值點為\(x_0=-2a\),
由\(f(x_1 )=f(x_0 )=1+4a^3\)可得\(x_1=a\),
則\(x_1⋅f(x_1+x_0 )=af(-a)=a(2a^3+1)=2a^4+a\),
設\(g(a)=2a^4+a(a<0)\),\(g'(a)=8a^3+1\),
當\(a \in\left(-\infty,-\dfrac{1}{2}\right)\),\(g'(a)<0\); \(a \in\left(-\dfrac{1}{2},+\infty\right)\),\(g'(a)>0\)
所以\(g(a)\)在\(\left(-\infty,-\dfrac{1}{2}\right)\)上單調遞減,在\(\left(-\dfrac{1}{2},+\infty\right)\)上單調遞增,
故\(g(a)_{\min }=g\left(-\dfrac{1}{2}\right)=-\dfrac{3}{8}\).
故選:\(B\). -
答案 \((-2,0)\)
解析 \(f'(x)=m\cdot e^x-2x+2(m<0)\),
所以\(f'(x)\)在\((0,1)\)上為減函式,
所以\(\left\{\begin{array}{l} f^{\prime}(0)=m+2>0 \\ f^{\prime}(1)=m e<0 \end{array}\right.\),解得\(-2<m<0\),
故答案為:\((-2,0)\). -
答案 \(3\)
解析 \(\because\)函式\(f(x)=x^3+2ax^2+x+1\), \(\therefore f'(x)=3x^2+4ax+1\),
又\(x_1\)和\(x_2\)是函式\(f(x)=x^3+2ax^2+x+1\)的兩個極值點,
則\(x_1\)和\(x_2\)是方程\(3x^2+4ax+1=0\)的兩根,
故 \(x_1+x_2=-\dfrac{4 a}{3}\), \(x_1 \cdot x_2=\dfrac{1}{3}\),
又\(x_2-x_1=2\),則\((x_1-x_2 )^2=(x_1+x_2 )^2-4x_1 x_2=4\),
即 \(\dfrac{16 a^2}{9}-\dfrac{4}{3}=4\),則\(a^2=3\). -
答案 (1)\(y=2x^3+3x^2-12x+1\);(2) 極大值\(21\), 極小值\(-6\).
解析 (1)\(f'(x)=6x^2+6x+a\), \(k_{\text {切 }}=f^{\prime}(0)=a\),
又因為切線方程為\(y=-12x+1\),
所以 \(k_{\text {切 }}=-12\),得\(a=-12\),
因為切點在切線上也在曲線上,
所以\(\left\{\begin{array}{l} f(0)=-12 \times 0+1=1 \\ f(0)=b \end{array}\right.\),所以\(b=1\),
所以\(f(x)\)的解析式為\(y=2x^3+3x^2-12x+1\).
(2)\(f(x)\)定義域為\(R\),\(f'(x)=6x^2+6x-12\)
令\(f'(x)=0\)得,\(x=-2\)或\(1\),
所以在\((-∞,-2)\),\((1,+∞)\)上單調遞增,在\((-2,1)\)上單調遞減,
所以\(f(x)_{\text {極大值 }}=f(-2)=21\), \(f(x)_{\text {極小植 }}=f(1)=-6\). -
答案 (1) 在\((-∞,-1)\),\((1,+∞)\)上單調遞增,在\((-1,1)\)上單調遞減;
(2) 當\(a=-1\)時,無極值;當\(a>-1\)時,極大值為\(\dfrac{1}{2} a+\dfrac{1}{6}\),極小值為\(-\dfrac{1}{6} a^3+\dfrac{3}{2} a^2\);
當\(a<-1\)時,極小值為\(\dfrac{1}{2} a+\dfrac{1}{6}\),極大值為\(-\dfrac{1}{6} a^3+\dfrac{3}{2} a^2\).
解析 (1)當\(a=1\)時,\(f(x)= \dfrac{1}{3} x^3-x\),\(\therefore f'(x)=x^2-1\),
令\(f'(x)=0\),解得\(x=1\),或\(x=-1\),
當\(f'(x)>0\)時,即\(x>1\),或\(x<-1\)時,函式為增函式,
當\(f'(x)<0\)時,即\(-1<x<1\),函式為減函式,
\(\therefore f(x)\)\(在(-∞,-1)\),\((1,+∞)\)上單調遞增,在\((-1,1)\)上單調遞減;
(2) \(f'(x)=x^2-(a-1)x-a=(x-a)(x+1)\),
令\(f'(x)=0\),解得\(x=-1\)或\(x=a\),
①當\(a=-1\)時,\(f'(x)⩾0\)恆成立,\(\therefore f(x)\)單調遞增,函式無極值,
②當\(a>-1\)時,
當\(f'(x)>0\)時,即\(x>a\),或\(x<-1\)時,函式為增函式,
當\(f'(x)<0\)時,即\(-1<x<a\),函式為減函式,
\(\therefore\)當\(x=-1\)時,函式有極大值,極大值為\(f(-1)=-\dfrac{1}{3}-\dfrac{1}{2}(a-1)+a=\dfrac{1}{2} a+\dfrac{1}{6}\),
當\(x=a\)時,函式有極小值,極小值為\(f(a)=\dfrac{1}{3} a^3-\dfrac{1}{2}(a-1) a^2+a^2=-\dfrac{1}{6} a^3+\dfrac{3}{2} a^2\),
③當\(a<-1\)時,
當\(f'(x)>0\)時,即\(x>-1\),或\(x<a\)時,函式為增函式,
當\(f'(x)<0\)時,即\(a<x<-1\),函式為減函式,
\(\therefore\)當\(x=-1\)時,函式有極小值,極小值為\(f(-1)=\dfrac{1}{2} a+\dfrac{1}{6}\),
當\(x=a\)時,函式有極大值,極大值為\(f(a)=-\dfrac{1}{6} a^3+\dfrac{3}{2} a^2\).
綜上所述,當\(a=-1\)時,無極值;
當\(a>-1\)時,極大值為\(\dfrac{1}{2} a+\dfrac{1}{6}\),極小值為\(-\dfrac{1}{6} a^3+\dfrac{3}{2} a^2\);
當\(a<-1\)時,極小值為\(\dfrac{1}{2} a+\dfrac{1}{6}\),極大值為 \(-\dfrac{1}{6} a^3+\dfrac{3}{2} a^2\).
【B組---提高題】
1.(多選)如圖,已知直線\(y=kx+m\)與曲線\(y=f(x)\)相切於兩點,則\(F(x)=f(x)-kx\)有( )
A.\(1\)個極大值點,\(2\)個極小值點 \(\qquad \qquad \qquad \qquad\) B.\(2\)個零點 \(\qquad \qquad \qquad \qquad\)
C.\(0\)個零點 \(\qquad \qquad \qquad \qquad \qquad \qquad \qquad\) D.\(2\)個極小值點,無極大值點
2.已知函式\(f(x)=e^x-a\sin x\)在區間 \(\left(0, \dfrac{\pi}{3}\right)\)上有極值,則實數\(a\)的取值範圍是\(\underline{\quad \quad}\) .
3.討論函式\(f(x)=x\ln x- \dfrac{1}{2} x^2+(a-1)x(a∈R)\)的極值點的個數.
參考答案
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答案 \(AC\)
解析 由原圖可知,\(k<0\),\(m>0\),設原圖中的兩切點橫座標為\(a\),\(b\).
再在同一座標系中做出\(y=f(x)\)與\(y=kx\)的圖象如圖:
由圖可知,\(y=f(x)\)與\(y=kx\)沒有公共點,
故函式\(F(x)\)沒有零點.
直線\(x=n\)與\(y=f(x)\)、\(y=kx\)分別交於點\(A\)、\(B\),
則\(F(x)\)的函式值可以理解為線段\(AB\)長度;
由圖可知:當\(x∈(-∞,a)\)時,\(F(x)\)單調遞減;當\(x∈(a,c)\),\(F(x)\)單調遞增;
當\(x∈(c,b)\)時,\(F(x)\)單調遞減;當\(x∈(b,+∞)\)時,\(F(x)\)單調遞增.
故\(a\),\(b\)是函式\(F(x)\)的極小值點,\(c\)是\(F(x)\))的極大值點.
故選:\(AC\). -
答案 \(\left(1,2 e^{\frac{\pi}{3}}\right)\)
解析 \(f(x)=e^x-\operatorname{asin} x\), \(x \in\left(0, \dfrac{\pi}{3}\right)\),
則\(f'(x)=e^x-a\cos x\)
由題意得\(e^x-a\cos x=0\)在 \(\left(0, \dfrac{\pi}{3}\right)\)上有解,
即\(a=\dfrac{e^x}{\cos x}\)在 \(\left(0, \dfrac{\pi}{3}\right)\)上有解,
記\(a=\dfrac{e^x}{\cos x}\), \(x \in\left(0, \dfrac{\pi}{3}\right)\),
則\(g^{\prime}(x)=\dfrac{e^x(\sin x+\cos x)}{\cos ^2 x}\),
當\(x \in\left(0, \dfrac{\pi}{3}\right)\)時,\(g'(x)>0\),\(g(x)\)遞增,
而\(g(0)=1\), \(g\left(\dfrac{\pi}{3}\right)=2 e^{\frac{\pi}{3}}\),故 \(1<a<2 e^{\frac{\pi}{3}}\). -
答案 當\(a≤1\)時,\(f(x)\)沒有極值點;當\(a>1\)時,\(f(x)\)有\(2\)個極值點.
解析 \(f'(x)=\ln x-x+a\), \(f^{\prime \prime}(x)=\dfrac{1}{x}-1=\dfrac{1-x}{x}\),
當\(x∈(0,1)\)時,\(f'' (x)>0\),\(f'(x)\)單調遞增;
當\(x∈(1,+∞)\)時,\(f'' (x)<0\),\(f'(x)\)單調遞減,
所以當\(x=1\)時,\(f'(x)\)有極大值,\(f'(1)=a-1\),
當\(a≤1\)時,\(f'(1)≤0\),
所以\(f(x)\)在\((0,+∞)\)上單調遞減,此時\(f(x)\)無極值,
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\(0<( \dfrac{1}{e})^{a+1}<x_1<1<x_2<e^a\),\(f'(x_1)=f'(x_2)=0\),
當\(x∈(0,x_1)\)時,\(f(x)\)單調遞減,當\(x∈(x_1,x_2)\)時,\(f(x)\)單調遞增,
當\(x∈(x_2,+∞)\)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.
【C組---拓展題】
1.已知函式 \(f(x)=\dfrac{e^x}{x}+k(\ln x-x)\),若\(x=1\)是函式\(f(x)\)的唯一極值點,則實數\(k\)的取值範圍是\(\underline{\quad \quad}\).
參考答案
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答案\((-∞,e]\)
解析 \(\because\)函式 \(f(x)=\dfrac{e^x}{x}+k(\ln x-x)\)的定義域是\((0,+∞)\),
\(\therefore f^{\prime}(x)=\dfrac{e^x(x-1)}{x^2}+\dfrac{k(1-x)}{x}=\dfrac{\left(e^x-k x\right)(x-1)}{x^2}\) ,
\(x=1\)是函式\(f(x)\)的唯一一個極值點,
\(\therefore x=1\)是導函式\(f'(x)=0\)的唯一根,
\(\therefore e^x-kx=0\)在\((0,+∞)\)無變號零點,
令\(g(x)=e^x-kx\),則\(g'(x)=e^x-k\),
①\(k≤0\)時,\(g'(x)>0\)恆成立,\(g(x)\)在\((0,+∞)\)時單調遞增,
\(g(x)\)的最小值為\(g(0)=1\),\(g(x)=0\)無解,
②\(k>0\)時,\(g'(x)=0\)有解,為:\(x=\ln k\),
\(0<x<\ln k\)時,\(g'(x)<0\),\(g(x)\)單調遞減,
\(\ln k<x\)時,\(g'(x)>0\),\(g(x)\)單調遞增,
\(\therefore g(x)\)的最小值為\(g(\ln k)=k-k\ln k\),
\(\therefore k-k\ln k>0\),\(\therefore k<e\),
畫出函式\(y=e^x\)和\(y=ex\)的圖象,如圖示:
由\(y=e^x\)和\(y=ex\)圖象,它們切於\((1,e)\),
綜上所述\(k≤e\),
故答案為:\((-∞,e]\).