-1/6-1/12-1/20-1/30……-1/420
问题描述:
-1/6-1/12-1/20-1/30……-1/420
1/6-1/12-1/20-1/30……-1/420
不要只写答案
答
-1/(2*3)-1/(3*4)-1/(4*5)-1/(5*6)-.-1/(19*20)-1/(20*21)=-(1/2-1/3)-(1/3-1/4)-(1/4-1/5)-...-(1/19-1/20)-(1/20-1/21)=-1/2+1/3-1/3+1/4-1/4+1/5-...-1/19+1/20-1/20+1/21=-1+1/21=-20/21