1/3-1/5=2/3*5,计算1/3*5+1/5*7+1/7*9+…+1/19*21
问题描述：
1/3-1/5=2/3*5,计算1/3*5+1/5*7+1/7*9+…+1/19*21
急
答
1/3*5+1/5*7+1/7*9+…+1/19*21
=(1/3-1/5+1/5-1/7+...+1/19-1/21)/2
=(1/3-1/21)/2
=(2/7)/2
=1/7