1/10×11+1/11×12+……1/19×21+1/20

问题描述:

1/10×11+1/11×12+……1/19×21+1/20

(1/10-1/11)+(1/11-1/22)+...+(1/18-1/19)(1/19-1/20)+1/20=1/10

原式=(1/10-1/11)+(1/11-1/12)+.+(1/19-1/20)=1/10-1/20=1/20