1/(12*13)+1/13*14)--1/(19*20)值=
问题描述：
1/(12*13)+1/13*14)--1/(19*20)值=
答
原式=(1/12-1/13)+(1/13-1/14)-(1/19-1/20)
=1/12-1/14-1/19+1/20
=1/84-1/380
=37/3990