5.3.1 (3) 函式的單調性(運用)
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基礎知識
函式單調性與導數
在某個區間\((a ,b)\)內,若\(f'(x)>0\),則函式\(y=f(x)\)在這個區間內單調遞增;
若\(f'(x)<0\) ,則函式\(y=f(x)\)在這個區間內單調遞減.
解釋
(1) 如下圖,導數\(f' (x_0)\)
在\(x=x_0\)處,\(f' (x_0 )>0\),切線是“左下右上”的上升式,函式\(f(x)\)的圖象也是上升的,函式\(f(x)\))在\(x=x_0\)附近單調遞增;
在\(x=x_1\)處,\(f' (x_1 )<0\),切線是“左上右下”的下降式,函式\(f(x)\)的圖象也是下降的,函式\(f(x)\)在\(x=x_1\)附近單調遞減.
(2) 若函式\(y=f(x)\)在某個區間\((a ,b)\)內單調遞增,則\(∀x∈(a ,b)\) ,\(f' (x)≥0\)(含等號)恆成立,但不存在一區間\((c ,d)⊆(a ,b)\)
假如存在一區間\((c ,d)⊆(a ,b)\)內使得\(f' (x)=0\),那原函式\(y=f(x)\)在區間\((c ,d)\)內恆等於一個常數,即\(f(x)=m\)(\(m\)是個常數),則原函式不可能在\((a ,b)\)內單調遞增.
\(\qquad \qquad\)
函式\(y=f(x)\)在某個區間\((a ,b)\)內單調遞減有類似結論!
(3)導函式“穿線圖”與原函式“趨勢圖”
① 導函式“穿線圖”關注導函式在各區間的正負,故特別注意函式與x軸的交點情況,
如\(f'(x)=x\)與\(f' (x)=e^x-1\)的“穿線圖”視為一樣的,它們在\((-∞,0)\)
\(\qquad \qquad\)
② 原函式“趨勢圖”僅關注函式在各區間上的單調性,沒顧及其最值或曲線形狀等,
如由導函式\(f' (x)=x-1\)的“穿線圖”易得原函式\(y=f(x)\)在\((-∞,0)\)上遞減,在\((0,+∞)\)上為遞增,趨勢圖可如下圖,
③ 後面涉及到函式單調性均可通過分析導函式“穿線圖”得出原函式的單調性.
基本方法
【題型1】已知函式單調性求引數範圍
【典題1】 若函式\(f(x)=x^3-3kx+1\)在區間\((1,+∞)\)上單調遞增,則實數\(k\)的取值範圍是( )
A.\((-∞,1)\) \(\qquad \qquad \qquad\) B.\((-∞,1]\) \(\qquad \qquad \qquad\) C.\([-1,+∞)\) \(\qquad \qquad \qquad\) D.\([1,+∞)\)
解析 因為\(f(x)=x^3-3kx+1\),所以\(f'(x)=3x^2-3k\),
當\(k⩽0\)時,\(f'(x)⩾0\)恆成立,\(f(x)\)在\(R\)單調遞增,滿足題意;
當\(k>0\)時,令\(f'(x)=3x^2-3k⩾0\),
則 \(x \leqslant-\sqrt{k}\)或\(x⩾\sqrt{k}\),
因為\(f(x)\)在區間\((1,+∞)\)上單調遞增,
所以\(\sqrt{k}⩽1\),即\(0<k⩽1\),
綜上所述,實數\(k\)的取值範圍是\((-∞,1]\).
故選:\(B\).
點撥 \(f(x)\)在\((a,b)\)上遞增\(⇒f'(x)≥0\)在\((a,b)\)上恆成立,等號是否成立有時需要檢驗; \(f(x)\)在\((a,b)\)上遞減\(⇒f'(x)≤0\)在\((a,b)\)上恆成立,等號是否成立有時需要檢驗.
【鞏固練習】
1.若函式\(f(x)=(x^2-4ax+2) e^x\)在\(R\)上單調遞增,則\(a\)的取值範圍是\(\underline{\quad \quad}\).
2.已知函式\(f(x)=\sin x+a\cos x\)在區間\(\left(\dfrac{\pi}{4}, \dfrac{\pi}{2}\right)\)上是減函式,則實數\(a\)的取值範圍為\(\underline{\quad \quad}\) .
3.已知函式\(f(x)=(x-a)\ln x\),\(a∈R\).若函式\(f(x)\)在\((0,+∞)\)上為增函式,則\(a\)的取值範圍\(\underline{\quad \quad}\) .
參考答案
-
答案 \(\left[-\dfrac{1}{2}, \dfrac{1}{2}\right]\)
解析 對函式求導:\(f'(x)=(x^2-4ax+2x+2-4a)⋅e^x\),
由已知有\(f'(x)≥0\)在\(R\)上恆成立,
又因為\(e^x>0\)恆成立,故僅需\(x^2+(2-4a)x+2-4a⩾0\)恆成立,
故\(△=(2-4a)^2-4(2-4a)⩽0\),解得\(-\dfrac{1}{2} ⩽a⩽\dfrac{1}{2}\) . -
答案 \([1,+∞)\)
解析 由題意得\(f'(x)=\cos x-a\sin x⩽0\)在區間\(\left(\dfrac{\pi}{4}, \dfrac{\pi}{2}\right)\)上恆成立,
所以 \(a \geqslant \dfrac{\cos x}{\sin x}=\dfrac{1}{\tan x}\)在區間\(\left(\dfrac{\pi}{4}, \dfrac{\pi}{2}\right)\)上恆成立,
因為當\(x \in\left(\dfrac{\pi}{4}, \dfrac{\pi}{2}\right)\)時,\(0<\dfrac{1}{\tan x}<1\),所以\(a⩾1\). -
答案 \(\left(-\infty,-e^{-2}\right]\)
解析 函式\(f(x)=(x-a)\ln x\),\(a∈R\),則\(f^{\prime}(x)=\ln x+1-\dfrac{a}{x}\),
函式\(f(x)\)在\((0,+∞)\)上為增函式,轉化為\(f'(x)⩾0\)在\((0,+∞)\)上恆成立,
即\(a⩽x\ln x+x\)在\((0,+∞)\)上恆成立,
令\(g(x)=x\ln x+x\),\(x∈(0,+∞)\),則\(g'(x)=\ln x+2\),
由\(g'(x)=0\)得\(x=e^{-2}\),由\(g'(x)>0\)得\(x>e^{-2}\),由\(g'(x)<0\)得\(0<x<e^{-2}\),
\(\therefore g(x)\)在\((0,e^{-2})\)上單調遞減,在\((e^{-2},+∞)\)上單調遞增,
\(\therefore\)當\(x=e^{-2}\)時,\(g(x)\)取得極小值也是最小值,且\(g(e^{-2})=-e^{-2}\),
\(\therefore a⩽-e^{-2}\),
故實數\(a\)的取值範圍為\((-∞,-e^{-2}]\).
【題型2】比較大小
【典題1】 若\(1<x<y<2\),則( )
A.\(e^x+3y<e^y+3x\) \(\qquad \qquad \qquad \qquad \qquad \qquad\) B.\(e^x+3y>e^y+3x\)
C.\(x^3+3y^2<y^3+3x^2\) \(\qquad \qquad \qquad \qquad \qquad \qquad\) D.\(x^3+3y^2>y^3+3x^2\)
解析 令\(f(x)=e^x-3x\),\(x∈(1,2)\),
則\(f'(x)=e^x-3\),由\(f'(x)=0\),可得\(x=\ln 3\),
所以當\(x∈(1,\ln 3)\)時,\(f'(x)<0\),\(f(x)\)單調遞減,
當\(x∈(\ln 3,2)\)時,\(f'(x)>0\),\(f(x)\)單調遞增,
所以當\(1<x<y<\ln 3\)時,\(e^x-3x>e^y-3y\),即\(e^x+3y>e^y+3x\),
當\(\ln 3<x<y<2\)時,\(e^x-3x<e^y-3y\),
即\(e^x+3y<e^y+3x\),故\(A\),\(B\)錯誤;
令\(g(x)=x^3-3x^2\),\(x∈(1,2)\),
則\(f'(x)=3x^2-6x=3x(x-2)<0\),
所以\(g(x)\)在\((1,2)\)上單調遞減,
因為\(1<x<y<2\),所以\(x^3+3y^2>y^3+3x^2\),故\(C\)錯誤,\(D\)正確.
故選:\(D\).
點撥 需要根據不等式建構函式,再通過函式單調性判斷大小.
【鞏固練習】
1.若\(x, y \in\left[-\dfrac{\pi}{2}, \dfrac{\pi}{2}\right]\),且\(x\sin x-y\sin y>0\),則下列不等式一定成立的是( )
A.\(x<y\) \(\qquad \qquad \qquad \qquad\) B.\(x>y\) \(\qquad \qquad \qquad \qquad\) C.\(|x|<|y|\) \(\qquad \qquad \qquad \qquad\) D.\(|x|>|y|\)
2.若對於任意的\(0<x_1<x_2<a\),都有 \(\dfrac{\ln x_1}{x_1}-\dfrac{\ln x_2}{x_2}<\dfrac{1}{x_2}-\dfrac{1}{x_1}\),則\(a\)的最大值為( )
A.\(2e\) \(\qquad \qquad \qquad \qquad\) B.\(e\) \(\qquad \qquad \qquad \qquad\) C.\(1\) \(\qquad \qquad \qquad \qquad\) D.\(\dfrac{1}{2}\)
3.若\(0<x_1<x_2<1\),則下列結論正確的是( )
A.\(\ln \dfrac{x_1}{x_2}<e^{x_1}-e^{x_2}\) \(\qquad \qquad \qquad \qquad\) B. \(\ln \dfrac{x_1}{x_2}>e^{x_1}-e^{x_2}\)
C.\(\dfrac{x_1}{x_2}>\sqrt{e^{x_1-x_2}}\) \(\qquad \qquad \qquad \qquad \qquad\) D. \(\dfrac{x_1}{x_2}<\sqrt{e^{x_1-x_2}}\)
參考答案
-
答案 \(D\)
解析 令\(f(x)=x\sin x\),\(x \in\left[-\dfrac{\pi}{2}, \dfrac{\pi}{2}\right]\),則\(f(x)\)為偶函式,
當\(x>0\)時,\(f'(x)=\sin x+x\cos x>0\),即\(f(x)\)在\(\left[0, \dfrac{1}{2} \pi\right]\)上單調遞增,
根據偶函式的對稱性可知,\(f(x)\)在\(\left[-\dfrac{1}{2} \pi, 0\right)\)上單調遞減,距離對稱軸越遠,函式值越大,
由\(x\sin x-y\sin y>0\),可得\(x\sin x>y\sin y\),即\(f(x)>f(y)\),
從而可得\(|x|>|y|\).
故選:\(D\). -
答案 \(C\)
解析 \(\because \dfrac{\ln x_1}{x_1}-\dfrac{\ln x_2}{x_2}<\dfrac{1}{x_2}-\dfrac{1}{x_1}\),\(\therefore \dfrac{\ln x_1+1}{x_1}<\dfrac{\ln x_2+1}{x_2}\),
據此可得函式\(f(x)=\dfrac{\ln x+1}{x}\)在定義域\((0,a)\)上單調遞增,
其導函式:\(f^{\prime}(x)=\dfrac{1-(\ln x+1)}{x^2}=-\dfrac{\ln x}{x^2} \geq 0\)在\((0,a)\)上恆成立,
據此可得:\(0<x≤1\),
即實數\(a\)的最大值為\(1\).
故選:\(C\). -
答案 \(D\)
解析 對於\(A\):由\(\ln \dfrac{x_1}{x_2}<e^{x_1}-e^{x_2}\) ,
得\(\ln x_1-\ln x_2<e^{x_1}-e^{x_2}\),得:\(e^{x_1}-\ln x_1>e^{x_2}-\ln x_2\),
令\(f(x)=e^x-\ln x\),則\(f(x_1)>f(x_2)\),
由函式\(f(x)=e^x-\ln x\),得:\(f'(x)=e^x-\dfrac{1}{x} \),\(f'(x)\)在\((0,1)\)遞增,
而\(f'(1)=e-1>0\),\(x→0\)時,\(f'(x)→-∞\),
故存在\(x_0∈(0,1)\),使得\(f'(x_0)=0\),
故\(f(x)\)在\((0,x_0)\)遞減,在\((x_0,1)\)遞增,
①若\(x_0<x_1<x_2<1\),則\(f(x_1 )<f(x_2 )\),
②若\(0<x_1<x_2<x_0\),則\(f(x_1 )>f(x_2 )\),
③若\(x_1<x_0<x_2\),則\(f(x_1 )\)和\(f(x_2 )\)無法比較大小,故\(A\)錯誤;
同理\(B\)錯誤;
對於\(C\):令\(g(x)=x^2 e^x\),\((0<x<1)\),
則\(g'(x)=e^x (x^2+2x)>0\),
故\(g(x)\)在\((0,1)\)遞增,由\(0<x_1<x_2<1\),得\(g(x_1 )<g(x_2 )\),
即\(x_1^2 e^{x_1}<x_2^2 e^{x_2}\),即\(\left(\dfrac{x_1}{x_2}\right)^2<\dfrac{e^{x_2}}{e^{x_1}}\),
故\(\dfrac{x_1}{x_2}<\sqrt{e^{x_2-x_1}}\),
故\(C\)錯誤,\(D\)正確;
故選:\(D\).
【題型3】比較數值大小
【典題1】 若 \(a=\dfrac{\ln 4}{4}\), \(b=\dfrac{\ln 5.3}{5.3}\), \(c=\dfrac{\ln 6}{6}\),則\(a\)、\(b\)、\(c\)的大小是( )
A.\(a<b<c\) \(\qquad \qquad\) B.\(c<b<a\) \(\qquad \qquad\) C.\(c<a<b\) \(\qquad \qquad\) D.\(b<a<c\)
解析 設 \(f(x)=\dfrac{\ln x}{x}\), \(f^{\prime}(x)=\dfrac{1-\ln x}{x^2}\),
令\(f'(x)>0\),可得\(0<x<e\),令\(f'(x)<0\),可得\(x>e\),
所以\(f(x)\)在\((0,e)\)上單調遞增,在\((e,+∞)\)上單調遞減,
因為\(e<4<5.3<6\),
所以\(f(4)>f(5.3)>f(6)\),
即 \(\dfrac{\ln 4}{4}>\dfrac{\ln 5.3}{5.3}>\dfrac{\ln 6}{6}\),即\(a>b>c\).
故選:\(B\).
點撥 需要數值的結構特點建構函式,再通過函式單調性判斷大小.
【典題2】 已知 \(a=e^{0.1}-1\),\(b=\sin 0.1\),\(c=\ln 1.1\),則( )
A.\(a<b<c\) \(\qquad \qquad\) B.\(b<c<a\) \(\qquad \qquad\) C.\(c<a<b\) \(\qquad \qquad\) D.\(c<b<a\)
解析 令\(f(x)=e^x-x-1\),則\(f'(x)=e^x-1\),
當\(x∈(0,+∞)\)時,\(f'(x)>0\),故\(f(x)\)在\((0,+∞)\)上是增函式,
故\(f(0.1)>f(0)\),即 \(e^{0.1}-0.1-1>0\),故\(a=e^{0.1}-1>0.1\),
令\(g(x)=\sin x-x\),則\(g'(x)=\cos x-1<0\)在\((0,1)\)上恆成立,
故\(g(x)=\sin x-x\)在\((0,1)\)上單調遞減,
故\(g(0.1)<g(0)\),即\(\sin 0.1-0.1<0\),即\(b=\sin 0.1<0.1\),
令\(h(x)=\ln (x+1)-\sin x\),
則 \(h^{\prime}(x)=\dfrac{1}{x+1}-\cos x=\dfrac{1-(x+1) \cos x}{x+1}\),
令\(m(x)=1-(x+1)\cos x\),
則\(m'(x)=-\cos x+(x+1)\sin x\),
易知\(m'(x)\)在\(\left(0, \dfrac{\pi}{6}\right)\)上是增函式,
且\(m^{\prime}\left(\dfrac{\pi}{6}\right)=-\dfrac{\sqrt{3}}{2}+\left(1+\dfrac{\pi}{6}\right) \dfrac{1}{2}=\dfrac{-6 \sqrt{3}+6+\pi}{12}<0\),
故\(m'(x)<0\)在\(\left(0, \dfrac{\pi}{6}\right)\)上恆成立,故\(m(x)\)在\(\left(0, \dfrac{\pi}{6}\right)\)上是減函式,
又\(\because m(0)=1-1=0\),故\(m(x)<0\)在\(\left(0, \dfrac{\pi}{6}\right)\)上恆成立,
故\(h'(x)<0\)在\(\left(0, \dfrac{\pi}{6}\right)\)上恆成立,故\(h(x)\)在\(\left(0, \dfrac{\pi}{6}\right)\)上是減函式,
故\(h(0.1)<h(0)=0\),即\(\ln 1.1-\sin 0.1<0\),即\(c<b\),
故\(c<b<a\),
故選:\(D\).
【鞏固練習】
1.已知 \(a=\dfrac{\ln \sqrt{2}}{4}\), \(b=\dfrac{1}{e^2}\), \(c=\dfrac{\ln \pi}{2 \pi}\),則\(a\)、\(b\)、\(c\)的大小關係為( )
A.\(a<c<b\) \(\qquad \qquad\) B. \(b<a<c\) \(\qquad \qquad\) C. \(a<b<c\) \(\qquad \qquad\) D. \(c<a<b\)
2.已知\(a=\dfrac{3}{\ln 3}\),\(b=e\), \(c=\dfrac{e^2}{2}\)(\(e\)為然對數的底數),則\(a\)、\(b\)、\(c\)的大小關係為( )
A.\(c>a>b\) \(\qquad \qquad\) B.\(c>b>a\) \(\qquad \qquad\) C.\(a>c>b\) \(\qquad \qquad\) D.\(b>c>a\)
3.設\(a=e^{-1}\) , \(b=\dfrac{1}{2} e^{-\frac{1}{2}}\),\(c=\ln 2\),則\(a\)、\(b\)、\(c\)的大小關係為( )
A.\(b<c<a\) \(\qquad \qquad\) B.\(a<b<c\) \(\qquad \qquad\) C. \(b<a<c\) \(\qquad \qquad\) D. \(c<a<b\)
4.設 \(a=\dfrac{10}{11}\), \(b=\dfrac{1}{e^{0.1}}\),\(c=0.9\),則( )
A.\(c<b<a\) \(\qquad \qquad\) B.\(c<a<b\) \(\qquad \qquad\) C.\(b<c<a\) \(\qquad \qquad\) D.\(a<c<b\)
參考答案
-
答案 \(C\)
解析 因為\(a=\dfrac{\ln \sqrt{2}}{4}=\dfrac{\ln 2}{8}=\dfrac{2 \ln 2}{16}=\dfrac{\ln 4}{16}\),\(b=\dfrac{1}{e^2}=\dfrac{\ln e}{e^2}\),\(c=\dfrac{\ln \pi}{2 \pi}=\dfrac{\ln \sqrt{\pi}}{\pi}\),
所以建構函式 \(f(x)=\dfrac{\ln x}{x^2}\),
則 \(f^{\prime}(x)=\dfrac{x-2 x \ln x}{x^4}=\dfrac{1-2 \ln x}{x^3}\),
令\(f'(x)>0\)得\(0<x<\sqrt{e}\);令\(f'(x)<0\)得\(x>\sqrt{e}\);
所以函式\(f(x)\)在\((0,\sqrt{e})\)上單調遞增,在\((\sqrt{e},+∞)\)上單調遞減,
因為\(\sqrt{e}<\sqrt{\pi}<e<4\),
所以\(f(\sqrt{\pi})>f(e)>f(4)\),即\(c>b>a\).
故選:\(C\). -
答案 \(A\)
解析 \(\because a=\dfrac{3}{\ln 3}\), \(b=e=\dfrac{e}{\ln e}\), \(c=\dfrac{e^2}{2}=\dfrac{e^2}{\ln e^2}\),
\(\therefore\)令\(f(x)=\dfrac{x}{\ln x}\),則 \(f^{\prime}(x)=\dfrac{\ln x-1}{(\ln x)^2}\),
\(\therefore\)當\(x∈(e,+∞)\)時,\(f'(x)>0\),
\(\therefore f(x)\)在\([e,+∞)\)上是增函式,
而\(a=\dfrac{3}{\ln 3}=f(3)\),\(b=e=\dfrac{e}{\ln e}=f(\mathrm{e})\),\(c=\dfrac{e^2}{2}=\dfrac{e^2}{\ln e^2}=f\left(e^2\right)\),
且\(e<3<e^2\),則\(c>a>b\),
故選:\(A\). -
答案 \(C\)
解析 設\(\text { 沒 } f(x)=x e^{-x}\) ,則 \(f^{\prime}(x)=(1-x) e^{-x}\),
當\(x≤1\)時,\(f'(x)≥0\),\(f(x)\)單調遞增,
當\(x>1\)時,\(f'(x)<0\),\(f(x)\)單調遞減,
\(\therefore f(1)>f(\dfrac{1}{2} )\),即\(a>b\),
\(\because c=\ln 2>\ln \sqrt{e}=\dfrac{1}{2}>\dfrac{1}{e}=a\),\(\therefore b<a<c\),
故選:\(C\). -
答案 \(A\)
解析 令\(f(x)=e^x-(x+1)\),則\(f'(x)=e^x-1\),
當\(x>0\)時,\(f'(x)>0\),當\(x<0\)時,\(f'(x)<0\),
\(\therefore\)當\(x=0\)時,\(f(x)\)取得最小值,即\(f(x)⩾f(0)=0\),
\(\therefore e^x⩾x+1\),\(\therefore e^{0.1}>0.1+1=1.1\),
\(\therefore \dfrac{1}{e^{0.1}}<\dfrac{1}{1.1}=\dfrac{10}{11}\),即\(b<a\),
\(\therefore \dfrac{1}{e^{0.1}}=e^{-0.1}>-0.1+1=0.9\),即\(b>c\),
綜上,可得\(c<b<a\).
故選:\(A\).
分層練習
【A組---基礎題】
1.已知函式\(f(x)=(x^2-4x+a) e^x\)在區間\([-2,3]\)上單調遞減,則實數\(a\)的取值範圍是( )
A.\((-∞,-4]\) \(\qquad \qquad\) B.\((-∞,5]\) \(\qquad \qquad\) C.\((-∞,1]\) \(\qquad \qquad\) D.\((-∞,-8]\)
2.已知\(a=\dfrac{\ln 3}{3}\),\(b=\dfrac{1}{e}\),\(c=\dfrac{\ln 5}{5}\),則以下不等式正確的是( )
A.\(c>b>a\) \(\qquad \qquad\) B.\(a>b>c\) \(\qquad \qquad\) C.\(b>a>c\) \(\qquad \ \qquad\) D.\(b>c>a\)
3.已知\(a=\dfrac{1}{e}\),\(b=\dfrac{\ln 5}{5}\),\(c=\dfrac{2}{5}\),則\(a\)、\(b\)、\(c\)的大小關係為( )
A.\(b<c<a\)\(\qquad \qquad\) B.\(c<a<b\) \(\qquad \qquad\) C.\(c<b<a\) \(\qquad \qquad\) D.\(b<a<c\)
4.下列不等式正確的是( )
A.\(\dfrac{\ln 2}{2}>\dfrac{\ln 4}{4}\) \(\qquad \qquad\) B.\(\dfrac{2 \ln 3}{3}>\ln 2\) \(\qquad \qquad\) C.\(e\ln 10>10\) \(\qquad \qquad\) D. \(2^{\sqrt{6}}>6\)
5.設\(a=e^{0.5}\),\(b=\dfrac{3}{e}\),\(c=\ln 3\),其中\(e\)為自然對數的底數,則\(a\)、\(b\)、\(c\)的大小關係是( )
A.\(c<a<b\) \(\qquad \qquad\) B. \(b<c<a\) \(\qquad \qquad\) C. \(a<c<b\) \(\qquad \qquad\) D. \(c<b<a\)
6.已知\(a=\ln \sqrt[3]{3}\),\(b=e^{-1}\),\(c=\ln (e+1)-1\),則( )
A.\(a<b<c\) \(\qquad \qquad\) B. \(c<a<b\) \(\qquad \qquad\) C. \(a<c<b\) \(\qquad \qquad\) D. \(b<c<a\)
7.若\(\ln x-\ln y<\dfrac{1}{\ln x}-\dfrac{1}{\ln y}(x>1, y>1)\),則( )
A. \(e^{y-x}>1\) \(\qquad \qquad\) B. \(e^{y-x}<1\) \(\qquad \qquad\) C. \(e^{y-x-1}>1\) \(\qquad \qquad\) D. \(e^{y-x-1}<1\)
8.若\(\alpha, \beta \in\left[-\dfrac{\pi}{2}, \dfrac{\pi}{2}\right]\),且\(α\sin α-β\sin β>\cos α-\cos β\),則下列結論中必定成立的是( )
A.\(α>β\) \(\qquad \qquad \qquad\) B.\(α>-β\) \(\qquad \qquad \qquad\) C.\(α<β\) \(\qquad \qquad \qquad\) D.\(|α|>|β|\)
9.已知 \(0<\alpha<\beta<\dfrac{\pi}{2}\),則下列不等式中恆成立的是( )
A.\(α^α<β^β\)\(\qquad \qquad \qquad\) B.\(α^α≤β^β\) \(\qquad \qquad \qquad\) C.\(α^β>β^α\) \(\qquad \qquad \qquad\) D.\(α^β<β^α\)
10.已知函式\(f(x)=\dfrac{1}{2} x^2+2ax-\ln x\),若\(f(x)\)在區間\([1,3]\)上單調遞增,則實數\(a\)的範圍為\(\underline{\quad \quad}\) .
參考答案
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答案 \(A\)
解析 因為函式\(f(x)=(x^2-4x+a) e^x\)在\([-2,3]\)上單調遞減,
所以\(f'(x)=(x^2-2x+a-4) e^x⩽0\)在\([-2,3]\)上恆成立,
則\(x^2-2x+a-4⩽0\)在\([-2,3]\)上恆成立,
即\(a \leqslant\left(-x^2+2 x+4\right)_{\text {min }}\),
又\(-x^2+2x+4\)的最小值為\(-4\),所以\(a⩽-4\),
故選:\(A\). -
答案 \(C\)
解析 令 \(f(x)=\dfrac{\ln x}{x}\),則 \(f^{\prime}(x)=\dfrac{1-\ln x}{x^2}\),
當\(x>e\)時,\(f'(x)<0\),函式單調遞減,
因為\(5>3>e\),所以\(f(5)<f(3)<f(e)\),
即 \(\dfrac{\ln 5}{5}<\dfrac{\ln 3}{3}<\dfrac{\ln e}{e}\),即\(b>a>c\).
故選:\(C\). -
答案 \(D\)
解析 \(\because a-c=\dfrac{1}{e}-\dfrac{2}{5}=\dfrac{5-2 e}{5 e}<0\),\(\therefore a<c\),
令 \(f(x)=\dfrac{\ln x}{x}\),則\(f^{\prime}(x)=\dfrac{1-\ln x}{x^2}\),
故當\(x∈(e,+∞)\)時, \(f^{\prime}(x)=\dfrac{1-\ln x}{x^2}<0\),
故\(f(x)\)在\([e,+∞)\)上是減函式,
而 \(a=\dfrac{1}{e}=f(\mathrm{e})\), \(b=\dfrac{\ln 5}{5}=f(5)\),
故\(b<a\),故\(b<a<c\),
故選:\(D\). -
答案 \(B\)
解析 因為 \(\dfrac{\ln 4}{4}=\dfrac{2 \ln 2}{4}=\dfrac{\ln 2}{2}\),\(A\)錯誤;
\(2\ln 3=\ln 9\),\(3\ln 2=\ln 8\),\(\ln 9>\ln 8\),故\(2\ln 3>3\ln 2\),
所以 \(\dfrac{2 \ln 3}{3}>\ln 2\),\(B\)正確;
令 \(f(x)=\dfrac{\ln x}{x}\),則 \(f^{\prime}(x)=\dfrac{1-\ln x}{x^2}\),
易得,當\(0<x<e\)時,\(f'(x)>0\),函式單調遞增,
當\(x>e\)時,\(f'(x)<0\),函式單調遞減,
故\(f(10)<f(e)\),
所以 \(\dfrac{\ln 10}{10}<\dfrac{1}{e}\),即\(e\ln 10<10\),\(C\)錯誤;
根據二次函式與冪函式性質可知,當\(2<x<4\)時,\(2^x<x^2\),
所以 \(2^{\sqrt{6}}<\sqrt{6}^2=6\),\(D\)錯誤.
故選:\(B\). -
答案 \(D\)
解析 易知\(a=e^{0.5}>\left(\dfrac{9}{4}\right)^{0.5}=1.5\),\(b=\dfrac{3}{e}<\dfrac{3}{2}=1.5\),\(c=\ln 3<\ln \sqrt{e^3}=1.5\),
令\(f(x)=\ln x-\dfrac{x}{e}\),則\(f^{\prime}(x)=\dfrac{1}{x}-\dfrac{1}{e}\),
故當\(x∈(e,+∞)\)時,\(f'(x)<0\),
故\(f(x)\)在\((e,+∞)\)上是減函式,
而\(f(e)=0\),故\(f(3)<0\),
即\(\ln 3-\dfrac{3}{e}<0\),即 \(\ln 3<\dfrac{3}{e}\),
故\(c<b<a\),
故選:\(D\). -
答案 \(B\)
解析 令 \(f(x)=\dfrac{\ln x}{x}(x \geq e)\),
則\(f^{\prime}(x)=\dfrac{1-\ln x}{x^2}<0\),即\(f(x)\)在\([e,+∞)\)上單調遞減,
所以\(a=\ln \sqrt[3]{3}=\dfrac{1}{3} \ln 3=f(3)\),\(b=e^{-1}=\dfrac{1}{e}=f(e)\),所以\(a<b\),
因為\(c=\ln (e+1)-1=\ln \dfrac{e+1}{e}=\ln \left(1+\dfrac{1}{e}\right)\),
因為\(\sqrt[3]{3}>1+\dfrac{1}{e}\),所以 \(\ln \sqrt[3]{3}>\ln \left(1+\dfrac{1}{e}\right)\),即\(a>c\),
綜上,\(b>a>c\).
故選:\(B\). -
答案 \(A\)
解析 依題意, \(\ln x-\dfrac{1}{\ln x}<\ln y-\dfrac{1}{\ln y}\),
令\(f(t)=t-\dfrac{1}{t}\),則 \(f^{\prime}(t)=1+\dfrac{1}{t^2}>0\),
\(\therefore\)函式\(f(t)\)在\(R\)上單調遞增,
\(\because \ln x-\dfrac{1}{\ln x}<\ln y-\dfrac{1}{\ln y}\),即 \(f(\ln x)<f(\ln y)\),
\(\therefore \ln x<\ln y\),
\(\therefore 1<x<y\),
\(\therefore y-x>0\),
\(\therefore e^{y-x}>e^0=1\).
故選:\(A\). -
答案 \(D\)
解析 不等式\(α\sin α-β\sin β>\cos α-\cos β\),可整理為\(α\sin α-\cos α>β\sin β-\cos β\),
令\(f(x)=x\sin x-\cos x\), \(x \in\left[-\dfrac{\pi}{2}, \dfrac{\pi}{2}\right]\),
上述不等式等價於\(f(α)>f(β)\),
\(\because f(-x)=(-x)\sin (-x)-\cos (-x)=x\sin x-\cos x=f(x)\),
\(\therefore f(x)\)為偶函式.
又\(f'(x)=2\sin x+x\cos x\),
\(\therefore\)當\(0<x≤\dfrac{\pi}{2} \)時,\(\sin x>0\),\(x\cos x≥0\),\(\therefore f'(x)>0\),
\(\therefore f(x)\)在 \(\left(0, \dfrac{\pi}{2}\right]\)上單調遞增,在 \(\left[-\dfrac{\pi}{2}, 0\right)\)上單調遞減.
結合\(f(x)\)的單調性和奇偶性可作出函式\(f(x)\)的大致草圖如下:
\(\because f(α)>f(β)\), \(\therefore |α|>|β|\).
故選:\(D\). -
答案 \(D\)
解析 建構函式 \(f(x)=\dfrac{\ln x}{x}\),則 \(f^{\prime}(x)=\dfrac{1-\ln x}{x}\),
令\(f'(x)>0\),解得\(0<x<e\),令\(f'(x)<0\),解得\(x>e\),
\(\therefore\)函式\(f(x)\)在(0,e)上單調遞增,在(e,+∞)上單調遞減,
\(\therefore\)函式\(f(x)\)在 \(\left(0, \dfrac{\pi}{2}\right)\)上單調遞增,
\(\therefore f(α)<f(β)\),即 \(\dfrac{\ln \alpha}{\alpha}<\dfrac{\ln \beta}{\beta}\),
\(\therefore β\ln α<α\ln β\),即\(\ln α^β<\ln β^α\),
\(\therefore α^β<β^α\),
故選:\(D\). -
答案 \([0,+∞)\)
解析 由題意知,\(f'(x)=x+2a-\dfrac{1}{x} ⩾0\)在\([1,3]\)上恆成立,
即\(2a⩾\dfrac{1}{x} -x\),
又函式\(y=\dfrac{1}{x} -x\)在\([1,3]\)上單調遞減,
所以當\(x=1\)時,函式\(y\)取得最大值,為\(0\),
所以\(2a≥0\),即\(a≥0\).
【B組---提高題】
1.若\(x,y∈(0,+∞)\),\(x+\ln x=e^y+\sin y\),則( )
A.\(\ln (x-y)<0\) \(\qquad \qquad\) B.\(\ln (y-x)>0\) \(\qquad \qquad\) C.\(x<e^y\) \(\qquad \qquad\) D.\(y<\ln x\)
2.已知\(a=8^{10}\),\(b=9^9\),\(c=10^8\),則\(a\),\(b\),\(c\)的大小關係為( )
A.\(b>c>a\) \(\qquad \qquad\) B.\(b>a>c\) \(\qquad \qquad\) C.\(a>c>b\) \(\qquad \qquad\) D.\(a>b>c\)
參考答案
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答案 \(C\)
解析 設\(f(x)=x-\sin x(x>0)\),則\(f'(x)=1-\cos x⩾0\)(不恆為零),
故\(f(x)\)在\((0,+∞)\)上為增函式,故\(f(x)>f(0)=0\),
所以\(x>\sin x\),故\(y>\sin y\)在\((0,+∞)\)上恆成立,
所以\(x+\ln x<e^y+y=e^y+\ln e^y\),
但\(g(x)=x+\ln x\)為\((0,+∞)\)上的增函式,
故\(x<e^y\),即\(\ln x<y\),
所以\(C\)成立,\(D\)錯誤.
取\(x=e\),考慮\(1+e=e^y+\sin y\)的解,若\(y⩾e+1\),
則\(e^y⩾e^{e+1}>5>e+2⩾1+e-\sin y\),矛盾,
故\(y<e+1\),即\(y-x<1\),
此時\(\ln (y-x)<0\),故\(B\)錯誤.
取\(y=1\),考慮\(x+\ln x=e+\sin 1\),
若\(x⩽2\),則\(x+\ln x⩽2+\ln 2<3<e+\dfrac{1}{2}<e+\sin 1\),矛盾,
故\(x>2\),此時\(x-y>1\),此時\(\ln (x-y)>0\),故\(A\)錯誤,
故選:\(C\). -
答案 \(D\)
解析 令\(f(x)=(18-x)\ln x\),
則\(f^{\prime}(x)=-\ln x+\dfrac{18}{x}-1\)在\(x⩾8\)時單調遞減,
又\(f^{\prime}(8)=\dfrac{5}{4}-\ln 8<\dfrac{5}{4}-\ln e^2<0\),
所以\(f'(x)<0\)在\(x⩾8\)時恆成立,
故\(f(x)\)在\(x⩾8\)時單調遞減,所以\(f(8)>f(9)>f(10)\).
所以\(10\ln 8>9\ln 9>8\ln 10\),故 \(8^{10}>9^9>10^8\).
故\(a>b>c\).
故選:\(D\).
【C組---拓展題】
1.若 \(x+\dfrac{3}{2} y-2=e^x+3 \ln \dfrac{y}{2}\),其中\(x>2\),\(y>2\),則( )
A.\(e^x<y\) \(\qquad \qquad\) B.\(2x>y\) \(\qquad \qquad\) C. \(4 e^{\frac{x}{2}}>y\) \(\qquad \qquad\) D.\(2e^x>y\)
2.若實數\(a\),\(b\)滿足 \(2 \ln a+\ln (2 b) \geq \dfrac{a^2}{2}+4 b-2\),則( )
A.\(a+b=\sqrt{2}+\dfrac{1}{4}\) \(\qquad \qquad\) B.\(a-2b=\sqrt{2}-\dfrac{1}{4} \) \(\qquad \qquad\) C.\(a^2+b>3\) \(\qquad \qquad\) D.\(a^2-4b<1\)
參考答案
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答案 \(D\)
解析 因為 \(x+\dfrac{3}{2} y-2=e^x+3 \ln \dfrac{y}{2}\),
所以 \(e^x-x=\dfrac{3}{2} y-2-3 \ln \dfrac{y}{2}=2\left(\dfrac{y}{2}-1-\ln \dfrac{y}{2}\right)+\dfrac{y}{2}-\ln \dfrac{y}{2}\),
令\(f(x)=x-1-\ln x\),則 \(f^{\prime}(x)=1-\dfrac{1}{x}=\dfrac{x-1}{x}\),
當\(x>1\)時,\(f'(x)>0\),\(f(x)\)為增函式,
所以\(f(x)>f(1)=0\),
因為\(y>2\),所以 \(\dfrac{y}{2}-1-\ln \dfrac{y}{2}>0\),
所以 \(e^x-x>\dfrac{y}{2}-\ln \dfrac{y}{2}=e^{\ln \dfrac{y}{2}}-\ln \dfrac{y}{2}\),\(x>2\), \(\ln \dfrac{y}{2}>0\),
令\(g(x)=e^x-x\),則\(g'(x)=e^x-1\),
當\(x>0\)時,\(g'(x)>0\),\(g(x)\)單調遞增,
所以 \(\text { 以 } e^x-x>\dfrac{y}{2}-\ln \dfrac{y}{2}=e^{\ln \dfrac{y}{2}}-\ln \dfrac{y}{2}\),等價於 \(x>\ln \dfrac{y}{2}\),
所以 \(e^x>\dfrac{y}{2}\),即\(2e^x>y\).
故選:\(D\). -
答案 \(A\)
解析 根據題意,若實數\(a\),\(b\)滿足 \(2 \ln a+\ln (2 b) \geq \dfrac{a^2}{2}+4 b-2\),
則\(a>0\)且\(b>0\),
又由 \(\dfrac{a^2}{2}+4 b-2 \geq 2 \times \sqrt{\dfrac{a^2}{2} \times 4 b}-2=2 \sqrt{2 a^2 b}-2\),當且僅當\(a^2=8b\)時等號成立,
則有 \(2 \ln a+\ln (2 b) \geq 2 \sqrt{2 a^2 b}-2\),變形可得 \(\ln \left(2 a^2 b\right)-2 \sqrt{2 a^2 b}+2 \geq 0\),
設 \(g(x)=\ln x-2 \sqrt{x}+2\),
則其導數 \(g^{\prime}(x)=\dfrac{1}{x}-\dfrac{1}{\sqrt{x}}=\dfrac{1-\sqrt{x}}{x}\),
當\(0<x≤1\)時,\(g'(x)≥0\),則\(g(x)\)在區間\((0,1]\)上為增函式,
當\(x≥1\)時,\(g'(x)≤0\),則\(g(x)\)在區間\([1,+∞)\)上為減函式,
則有\(g(x)≤g(1)=0\),
若\(\ln \left(2 a^2 b\right)-2 \sqrt{2 a^2 b}+2 \geq 0\),
即\(g(2a^2 b)≥0\),必有\(2a^2 b=1\),
又\(a^2=8b\),所以\(a=\sqrt{2}\),\(b=\dfrac{1}{4} \) ,
據此分析選項:對於\(A\),\(a+b=\sqrt{2}+\dfrac{1}{4} \),\(A\)正確;
對於\(B\),\(a-2b=\sqrt{2}-\dfrac{1}{2} \),\(B\)錯誤;
對於\(C\), \(a^2+b=\dfrac{9}{4}\),\(C\)錯誤;
對於\(D\),\(a^2-4b=1\),\(D\)錯誤;
故選:\(A\).