R U ①U R R =0~T:t2()=Uen (t-7)/r tT (t)=—U R U R T R UK R +_ C C u U u R 1 2 u C u t R U t=0 ~T ： /  ( ) t u R t Ue − = t=T ~ ： ( ) /  ( ) t T u R t Ue − − = − u R U-U t T u C u t R U-U u C t U-U T u R