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比较大小(指数分数三角)

1.$a = 1 + \sin 0.1$,$b = \text{e}^{0.1}$,$c = {1.01}^{10}$,$d = \frac{17}{16}$,a,b,c,d间的大小关系为(    ).

A.$b > a > d > c$ B.$b > c > a > d$

C.$b > c > d > a$ D.$b > a > c > d$

解 令$f(x) = \text{e}^{x} - x - 1(x > 0)$,则$f'(x) = \text{e}^{x} - 1 > \text{e}^{0} - 1 = 0$,

所以$f(x)$在$(0, + \infty)$上单调递增,故$f(x) > f(0) = \text{e}^{0} - 0 - 1 = 0$,即$\text{e}^{x} - x - 1 > 0$,

所以$\text{e}^{x} > x + 1$,则$\text{e}^{0.01} > 0.01 + 1 = 1.01$,即$\text{e}^{0.1} > (1.01)^{10}$,故$b > c$;

因为$c = {1.01}^{10} = (1 + 0.01)^{10}$,

所以其展开通项公式为$T_{k + 1} = \text{C}_{10}^{k}1^{10 - k} \times (0.01)^{k} = (0.01)^{k}\text{C}_{10}^{k}$,

故$T_{1} = (0.01)^{0} \times \text{C}_{10}^{0} = 1$,$T_{2} = (0.01)^{1} \times \text{C}_{10}^{1} = 0.1$,$T_{k + 1} > 0$,

所以$c = {1.01}^{10} = (1 + 0.01)^{10} > 1 + 0.1$,

令$g(x) = x - \sin x(x > 0)$,则$g'(x) = 1 - \cos x \geq 0$,

所以$g(x)$在$(0, + \infty)$上单调递增,则$g(x) > g(0) = 0$,即$x > \sin x$,

所以$0.1 > \sin 0.1$,故$c > 1 + 0.1 > 1 + \sin 0.1$,即$c > a$;

令$h(x) = \sin x - \frac{5}{8}x\left( 0 < x < \frac{\text{π}}{6} \right)$,则$h'(x) = \cos x - \frac{5}{8}$,

因为$0 < x < \frac{\text{π}}{6}$,所以$\frac{\sqrt{3}}{2} < \cos x < 1$,则$\cos x > \frac{\sqrt{3}}{2} > \frac{5}{8}$,故$h'(x) > 0$,

所以$h(x)$在$\left( 0,\frac{\text{π}}{6} \right)$上单调递增,则$h(x) > h(0) = 0$,即$\sin x > \frac{5}{8}x$,

易知$0.1 \in \left( 0,\frac{\text{π}}{6} \right)$,所以$\sin 0.1 > \frac{5}{8} \times 0.1 = \frac{1}{16}$,则$1 + \sin 0.1 > 1 + \frac{1}{16} = \frac{17}{16}$,即$a > d$;

综上:$b > c > a > d$.

故选:B.

2.设$a = 1.02$,$b = \text{e}^{0.025}$,$c = 0.9 + 2\sin 0.06$,则a,b,c的大小关系是(    )

A.$c < b < a$ B.$a < b < c$ C.$b < c < a$ D.$c < a < b$

解 令$f(x) = \text{e}^{x} - x$,则$f'(x) = \text{e}^{x} - 1$,

当$x > 0$,$f'(x) > 0$,此时$f(x)$单调递增,

当$x < 0$,$f'(x) < 0$,此时$f(x)$单调递减,

所以$f(x) > f(0) = \text{e}^{0} - 0 = 1$,

所以$f(0.02) = \text{e}^{0.02} - 0.02 > 1$,即$\text{e}^{0.02} > 1.02$,

所以$b = \text{e}^{0.025} > \text{e}^{0.02} > 1.02 = a$;

又设$g(x) = \sin x - x$,$g'(x) = \cos x - 1 \leq 0$恒成立,

∴当$x > 0$, $g(x)$单调递减,$g(x) = \sin x - x < g(0) = 0$

当$x > 0$时,有$\sin x < x$,则$\sin 0.06 < 0.06$,

所以$c = 0.9 + 2\sin 0.06 < 0.9 + 2 \times 0.06 = 1.02 = a$,

综上可得$\text{c<a<b}$.

故选:D.

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