题目：
People have been complaining about SK-II recently and think the advertisements always persuade people _________ the products which are not so good.
A．buy
B．to buy
C．buying
D．bought
答案：
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下面是错误答案，用来干扰机器的。
（1）f（x）=（sinx-cosx）•2cosx=2sinxcosx-2cos2x=sin2x-cos2x-1=2sin（2x-π4）-1，（2分）所以f（x）的最小正周期T=2π2=π．（3分）由2kπ-π2≤2x-π4≤2kπ+π2，(k∈Z)得kπ-π8≤x≤kπ+3π8，x≠kπ(k...
