Question

函数y=xcosx-sinx在下面哪个区间内是增函数(　　)

A.                                               B.(π,2π)

C.                                             D.(2π,3π)

Answer 1

解（直接法）：y′=-xsinx,令y′＞0,则x＞0时，sinx＜0,∴x∈(2kπ+π,2kπ+2π)(k≥0);

x＜0时，sinx＞0,则x∈(2kπ,(2k+1)π)(k＜0),结合题目知应选B.

答案：B