b.= 2 n T o/(r)sin ndx==5oxsinnxdr T xcos sinu 210 T n n 2 2 =--c0sm兀=2(-1)”+ ,(n=1,2,) n n f(x)=2(sin x-sin 2x +sin 3x-.) 3 n+1 ∑ SInx (00<x<+0;x≠土冗,+3元,…) 上页 圆  =  0 ( )sin 2 bn f x nxdx   =  0 sin 2 x nxdx  − +  = 2 0 ] cos sin [ 2 n nx n x nx = − n n cos 2 ( 1) , 2 +1 = − n n (n = 1,2, ) sin3 ) 3 1 sin2 2 1 f (x) = 2(sin x − x + x − sin . ( 1) 2 1 1   = + − = n n nx n (−  x  +; x  ,3, )