1/2*3+1/3*4+1/4*5+...+1/99*100
=(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+...+(1/99-1/100)
=1/2-1/3+1/3-1/4+1/4-1/5+...+1/99-1/100
=1/2-1/100
=49/100这是用什么规律？？列项1/n*(n+1)=1/n-1/(n+1)