1u1|2l233 123 D d +l+l-x 2)y1(x,1)=41coso(t x-20-d =Acos a(t+ )-丌 x-2l=d 2D 3)Vi(, t)=4 cos Q(t8 3) ) 2 2 ( , ) cos ( 1 2 1 1 u D u x l d y x t A t − − − = + − + + − ( , ) = cos ( − ) 1 1 1 u d l l x 2) y x t A t − − − = + ) 2 cos ( 1 1 u x l d A t 1u1 2u2 d l S D 1 S2 0 1 2 a x x 3 y 3u3 y1 y1 y1