3/1-5/1=3*5分之1*2应该是1/3-1/5=3*5分之1*2
3*5分之1+5*7分之1+7*9分之1+······+19*21分之1
=(1/2)×[(1/3-1/5)+(1/5-1/7)+(1/7-1/9)+……+(1/19-1/21)]
=(1/2)×(1/3-1/21)
=1/7