題目
Problem
Part I Multiple Choice
- A function f on R is odd if f(−x)=−f(x) for all x∈R while it is even if f(−x)=f(x) for all x∈R. Suppose that g and h are two functions on R.
(a) If g is an even function, then the composite function h∘g must be even.
(b) If g is an odd function and limx→0+g(x)=3, then the limit limx→0−g(x) must be −3.
(c) If g is an even function and limx→0+g(x)=3, then the limit limx→0−g(x) must be 3.
Among the above three statements, how many of them are true?
A) Only one.
B) Only two.
C) All of them.
D) None of them.
解答
解法一
思路
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逐句直接使用偶、奇函數的定義,尤其注意左右極限可藉 x↦−x 互換。
答題過程
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若 g 是偶函數,則
(h∘g)(−x)=h(g(−x))=h(g(x))=(h∘g)(x),
故 (a) 正確。
若 g 是奇函數,令 x=−t(t→0+),則
x→0−limg(x)=t→0+limg(−t)=−t→0+limg(t)=−3,
故 (b) 正確。若 g 是偶函數,同樣代入 x=−t 可得左極限等於右極限 3,所以 (c) 亦正確。
三句皆正確,選 C。