x,)=∑( 2k+1)tat COS +B )s(2k+1)mD 2 f Sib(2k+lrat 27 2l ∑Asn (2k+1)zx 21 ∑ (2k+1)ma B, sin 2k+1)mt(2k+1)zo SIn B.=0 2 2l IS防(2k+1)mx. 2 (2 1) sin 2 0 dx l k x x l A l k  +  = −  . 2 (2 1) sin 0 l k x x Ak k   + − =   = . 2 (2 1) sin 2 (2 1) sin 2 (2 1) 0 0 l k x l k at B l k a k k +  +  +  =   = Bk = 0 . 2 (2 1) )sin 2 (2 1) sin 2 (2 1) ( , ) ( cos 0 l k x l k at B l k at u x t Ak k k  +  +  + + =   =