A1=1/2A2=2/3A3=3/4……St=1/2+2/3+3/4+……+t/(t+1)=(1-1/2)+(1-1/3)+(1-1/4)+……+[1-1/(t+1)]=t-(1/2+1/3+1/4+……1/(t+1))= 剩下的不会了,想出来再告诉你如果是乘还简单一点~~~~~