-1/10×11-1/11×12-1/12×13—……-1/19×20
=-(1/10×11+1/11×12+1/12×13—……+1/19×20)
=-(1/10-1/11+1/11-1/12+1/12-1/13+...+1/19-1/20)
=-(1/10-1/20)
=-1/20
公式1/n(n+1)=1/n-1/(n+1)