Question

1/10*11+1/11*12+...+1/19*20=?

Answer 1

1/10*11=1/10-1/11

1/11*12=1/11-1/12

……

所以原式=(1/10-1/11)+(1/11-1/12)+…+(1/19-1/20)=1/10-1/20=1/20