1/(10*11)=1/10-1/11
1/10*11+1/11*12+1/12*13.1/19*20
=(1/10-1/11)+(1/11-1/12)+(1/12-1/13)+...+(1/18-1/19)+(1/19-1/20)
=1/10-1/20
=1/20