+2o2a.=-dg(0 dt i.(o,0)=0 i(@,t)=-a25g(x)e-ndx x.()cosoxdo ()cosxd = =(g(sy--b小cusoxdw 44 2 2 2 ˆ ˆ ( ) ˆ ( ,0) 0 c c c du a u a g t dt u            2 2 2 ( ) 0 ˆ ( , ) ( ) t a t x u t a g x e dx c          2 2 0 2 ( ) 0 0 2 ( , ) ( , )cos ˆ 2 cos ( ) c t a t x u x t u a g x e d t x x x d d                       2 2 2 ( ) 0 0 2 ( ) cos t a a t s g s e ds xd           