2π f(x) cos nn,(n=0,1,2,…) 0 或 2兀 f(rsin nxd, (n=1, 2,.) 兀0 傅里叶级数 U +2(a, cos nx+b, sin nx) H-=1 问题: f(x)条件?+∑( a. cos nr+ b sinn)     =  = =  =     2 0 2 0 ( )sin , ( 1,2, ) 1 ( )cos , ( 0,1,2, ) 1   b f x nxdx n a f x nxdx n n n 或 傅里叶级数 +  +  =1 0 ( cos sin ) 2 n an nx bn nx a 问题: +  +  =1 0 ( cos sin ) 2 ( ) ? n an nx bn nx a f x 条件