題目
Problem
Suppose that f(x)=ln(1+x2)+2x for x∈(−∞,∞) and h is a differentiable function defined on (−∞,∞) such that f(h(x))=x2+x. Which of the following statements is true?
(a) −∞<h′(0)≤0.
(b) 0<h′(0)≤1.
(c) 1<h′(0)≤2.
(d) 2<h′(0)<∞.
(e) h′(0) cannot be determined based on the given information.
解答
解法一
思路
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- 先由 x=0 找 h(0),再對合成等式微分。
答題過程
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f(0)=0 且 f′(u)=1+u22u+2>0,故 h(0)=0。微分 f(h(x))=x2+x:
f′(h(0))h′(0)=1⟹2h′(0)=1.
所以 h′(0)=21,答案為 (b)。