3。周期为2L的函数展开为 Fourier级数 T n x)cos (n=0,1,2,…) nTr f()sin dx, (n=1, 2, . f(x)=4+∑( nTr nTtr a cos +b, sin-) 2 H=13。周期为 2L 的函数展开为 Fourier 级数 ( )cos , ( 0,1,2, ) 1 =   = − dx n l n x f x l a l l n ( )sin , ( 1,2, ) 1 =   = − dx n l n x f x l b l l n ( cos sin ), 2 ( ) 1 0 l n x b l n x a a f x n n n  +  = +  =