下列以2π为周期的周期函数中，傅里叶级数是余弦级数的是( )． (A) (B)f(x)=xcosx -π＜x≤π (C)f(x)=x-x3 -π<x≤π (D)f(x)=|sinx| -π＜x≤π
A.
B.174.24.96:6088/Latex/latex.action?latex=Zih4KT1cbGVmdFx7XGJlZ2lue2FycmF5fXtjY30tMSwgJi1ccGkg77yceO%2B8nDAgXFwwLCAmeD0w%0D%0AIFxcMSwgJjDvvJx4XGxlcSBccGlcZW5ke2FycmF5fQ%3D%3D'