如果题目没写错
-1/10*11+1/11*12+1/12*13+1/13*14+···1/19*20
= -1/10+1/11+1/11-1/12++···+1/19-1/20
=-1/10+1/11+1/11-1/20
=2/11-3/20
如果是
1/10*11+1/11*12+1/12*13+1/13*14+···1/19*20
=1/10-1/20=1/20 中间相消去