(3)求bn ∫ f(x) sin ndx=“rc sin ndx 2 +>lakt cos kx sin ndx +be[ sin kx sin ndx]=b,It, T f()sin ndx (n T 傅里叶系数 f()cos nxd, (n=0, 1, 2,. T T f(sin ndx ,(n=1,2 T=兀(3) . 求bn    −  − = nxdx a f x nxdx sin 2 ( )sin 0 [ cos sin sin sin ] 1     −  −  = + a kx nxdx + bk kx nxdx n k = , n b   =  − b f x nxdx n ( )sin 1 (n = 1,2,3, ) 傅里叶系数      =  = =  =    −  − ( )sin , ( 1,2, ) 1 ( )cos , ( 0,1,2, ) 1   b f x nxdx n a f x nxdx n n n