1又1*3分之1+2又3*5分之1+3又5*7分之1+4又7*9分之1+...+10又19*21分之1
=(1+2+3+4+5+6+7+8+9+10)+(1/1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+······+1/19-1/20)÷2
=55+(1-1/20)÷2
=55+19/40
=55又19/40