-1/10*11-1/11*12-1/12*13-

-1/10*11-1/11*12-1/12*13-……-1/19*20

5个回答

原式=-(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