题目

The woman is very busy so she ______ watches TV. A . often B . always C . seldom D . almost 答案：C16．已知等差数列{an}中，${a_5}=\frac{π}{2}$若函数f（x）=sin2x-cosx-1，设cn=f（an），则数列{cn}的前9项和为（　　）A．0B．1C．9D．-9