25~26高三上·湖北12月多校联考·第8题

已知$a = \frac{1}{4}\cos\frac{1}{4}$,$b = \sin\frac{1}{4}$,$c = \ln\frac{4}{3}$,则( )
A. $c < b < a$
B. $c < a < b$
C. $a < b < c$
D. $a < c < b$

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C

令$f(x) = \tan x – x\left(0 < x < \frac{\pi}{2}\right)$,

则$f'(x) = \frac{1}{\cos^2 x} – 1 = \frac{1 – \cos^2 x}{\cos^2 x} = \tan^2 x > 0$,
$\therefore f(x)$在$\left(0, \frac{\pi}{2}\right)$上单调递增,$\therefore f\left(\frac{1}{4}\right) > f(0) = 0$,
即$\tan\frac{1}{4} > \frac{1}{4}$,$\therefore \frac{\sin\frac{1}{4}}{\cos\frac{1}{4}} > \frac{1}{4}$,

又$\cos\frac{1}{4} > 0$,$\therefore \sin\frac{1}{4} > \frac{1}{4}\cos\frac{1}{4}$,即$a < b$;

令$g(x) = \sin x + \ln(1 – x)\left(0 < x < 1\right)$,则$g'(x) = \cos x – \frac{1}{1 – x}$,
令$h(x) = g'(x)$,则$h'(x) = -\sin x – \frac{1}{(1 – x)^2} < 0$,
$\therefore g'(x)$在$(0, 1)$上单调递减,
$\therefore g'(x) < g'(0) = 0$,$\therefore g(x)$在$(0, 1)$上单调递减,
$\therefore g\left(\frac{1}{4}\right) < g(0) = 0$,即$\sin\frac{1}{4} < -\ln\frac{3}{4} = \ln\frac{4}{3}$,
$\therefore b < c$.

综上所述:$a < b < c$.

故选:C.

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