「2+∑,ckc+∑么smnk 2π 2 ∫f(x)dc (2)求a n (x) cos ndx=“/ t cos ndx 2 +∑ak- cos k cos nxdx+bk∫ sin kx cos ndx 2 cos nra dx nJ-兀 f(x)cos nxdx (n=1, 2,3,".) 丌兀dx a kxdx b kxdx a k k k k cos sin 2 1 1 0    −   = −  = − = + +       2 , 2 0 =   a   =  − a f (x)dx 1 0 (2) . 求an  =   −  − nxdx a f x nxdx cos 2 ( )cos 0 [ cos cos sin cos ] 1 +   +   −  −  = a kx nxdx bk kx nxdx n k =   − an nxdx 2 cos = , an    − a = f x nxdx n ( )cos 1 (n = 1,2,3, )