B,=0 k-1 8al (2k+1)2x Bal l(x,) ∑ 1)c05(2k+1)m (2k+1)mx z22(2k+1)2 2l 解1坐标原点置于杆左端 a2u=0 XX L以(x 0 0 t=0 t=0 0Bk = 0 2 2 1 (2 1) 8 ( 1)   + = − − k l A k k . 2 (2 1) sin 2 (2 1) cos (2 1) 8 ( 1) ( , ) 2 1 0 2 l k x l k at k l u x t k k     + + + − =  − =  解1 坐标原点置于杆左端 0 2 utt − a uxx = u(x,t) x=0 = 0 ux (x,t) x=2l = 0 u x ut t=0 = 0 t = − =0