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| 5.3.1 函数的单调性题目答案及解析如下,仅供参考!
选择性必修二
第五章 一元函数的导数及其应用
5.3 导数在研究函数中的应用
5.3.1 函数的单调性
设$a=\ln 1.1$,$b={{\text{e}}^{0.1}}-1$,$c=\tan 0.1$,$d=\dfrac{0.4}{\pi }$,则$(\qquad)$.
$a\\lt b\\lt c\\lt d$
","$a\\lt c\\lt b\\lt d$
","$a\\lt b\\lt d\\lt c$
","$a\\lt c\\lt d\\lt b$
"]设$a\left( x \right)=\ln \left( x+1 \right)$,$b\left( x \right)={{\text{e}}^{x}}-1$,$c\left( x \right)=\tan x$,$d\left( x \right)=\dfrac{4}{\pi }x$,易得$a\left( 0 \right)=b\left( 0 \right)=c\left( 0 \right)=d\left( 0 \right)$$.$
设$y=d\left( x \right)-b\left( x \right)=\dfrac{4}{\pi }x-{{\text{e}}^{x}}+1$,则令${y}'=\dfrac{4}{\pi }-{{\text{e}}^{x}}=0$有$x=\ln \dfrac{4}{\pi }$,故$y=d\left( x \right)-b\left( x \right)$在$\left( -\infty ,\ln \dfrac{4}{\pi } \right)$上单调递增$.$
①$\because {{\left( \dfrac{4}{\pi } \right)}^{10}}\gt {{\left( \dfrac{4}{3.2} \right)}^{10}}={{\left( \dfrac{5}{4} \right)}^{10}}={{\left( \dfrac{25}{16} \right)}^{5}}\gt {{\left( \dfrac{24}{16} \right)}^{5}}={{\left( \dfrac{3}{2} \right)}^{5}}\gt \text{e}$,即${{\left( \dfrac{4}{\pi } \right)}^{10}}\gt \text{e}$,故$10\ln \dfrac{4}{\pi }\gt 1$,即$\ln \dfrac{4}{\pi }\gt 0.1$,故$d\left( 0.1 \right)-b\left( 0.1 \right)\gt d\left( 0 \right)-b\left( 0 \right)=0$,即$d\gt b$
②设$y=b\left( x \right)-c\left( x \right)={{\text{e}}^{x}}-1-\tan x$,则${y}'={{\text{e}}^{x}}-\dfrac{1}{{{\cos }^{2}}x}=\dfrac{{{\text{e}}^{x}}{{\cos }^{2}}x-1}{{{\cos }^{2}}x}$,设$f\left( x \right)={{\text{e}}^{x}}{{\cos }^{2}}x-1$,则${f}'\left( x \right)={{\text{e}}^{x}}\left( {{\cos }^{2}}x-2\sin x \right)={{\text{e}}^{x}}\left( -{{\sin }^{2}}x-2\sin x+1 \right)$
设$g\left( x \right)=x-\sin x$,则${g}'\left( x \right)=1-\cos x\ge 0$,故$g\left( x \right)=x-\sin x$为增函数,故$g\left( x \right)\ge g\left( 0 \right)=0$,即$x\ge \sin x$
故${f}'\left( x \right)\ge {{\text{e}}^{x}}\left( -{{x}^{2}}-2x+1 \right)={{\text{e}}^{x}}\left[ -{{\left( x+1 \right)}^{2}}+2 \right]$,当$x\in \left[ 0,0.1 \right]$时${f}'\left( x \right)\gt 0$, $f\left( x \right)={{\text{e}}^{x}}{{\cos }^{2}}x-1$为增函数,故$f\left( x \right)\ge {{\text{e}}^{0}}{{\cos }^{2}}0-1=0$,故当$x\in \left[ 0,0.1 \right]$时$y=b\left( x \right)-c\left( x \right)$为增函数,故$b\left( 0.1 \right)-c\left( 0.1 \right)\gt b\left( 0 \right)-c\left( 0 \right)=0$,故$b\gt c$
③设$y=c\left( x \right)-a\left( x \right)=\tan x-\ln \left( x+1 \right)$,${y}'=\dfrac{1}{{{\cos }^{2}}x}-\dfrac{1}{x+1}=\dfrac{x+{{\sin }^{2}}x}{\left( x+1 \right){{\cos }^{2}}x}$,易得当$x\in \left( 0,0.1 \right)$时${y}'\gt 0$,故$c\left( 0.1 \right)-a\left( 0.1 \right)\gt c\left( 0 \right)-a\left( 0 \right)=0$,即$c\gt a$
综上$d\gt b\gt c\gt a$.
故选:$\rm B$
| 5.3.1 函数的单调性题目答案及解析(完整版)