1/2+1/6+1/12+1/20+1/30+1/42……1/380
=1/(1*2)+1/(2*3)+1/(3*4)……1/（19*20）
=1/1-1/2+1/2-1/3+1/3-1/4……-1/19+1/19-1/20
=1-1/20
=19/20