(1-1/2)*(1-1/3)*(1-1/4)*(1-1/5)*.*(1-1/99)X(1-1/100)
=(1/2)*(2/3)*(3/4)*(4/5)*(5/6)...*(98/99)*(99/100)
=1/100(中间项的分子分母可以约掉)