2 =u(t)cos ntt 2TEsintcos ntt rJo Isin(n+1)t-sin(n-1)tdt e cos(n+1)t, cos(n-1)t T n+1 (n≠1) n 0 4E 当n=2k =11(2k)2-1li (k=1,2,…) 0 当n=2k+1 上页  =  0 ( )cos 2 a u t ntdt n   =  0 sin cos 2 E t ntdt   + − −  = 0 [sin(n 1)t sin(n 1)t]dt E     = + = −  − = 0, 2 1 , 2 [(2 ) 1] 4 2 n k n k k E 当 当 (k = 1,2, )        − − + + + −  = 1 0 cos( 1) 1 cos( 1) n n t n E n t (n  1)