1/(12*13)+1/(13*14)+。。。。。+1/(19*20)=?另1/2+1/6+1/12+1/20+1/30+1/42=?

热心网友

1/(12*13)+1/(13*14)+。。。。。+1/(19*20)=1/12-1/13+......+1/19-1/20=1/301/2+1/6+1/12+1/20+1/30+1/42=1-1/2+1/2-1/3+1/3-1/4+...+1/6-1/7=6/7