1/(1×2)+1/(2×3)+1/(3×4)+……+1/(19×20)+1/(20×21)=?
巧算:1/1*2+1/2*3+1/3*4+……1/19*20+1/20*21=
1-2+3-4+5-6+……+19-20+21=?
巧算:1/1*2 + 1/2*3 + 1/3*4 +1/4*5 ……+ 1/19*20 +1/20*21 .
(1-1/2+1/3-1/4+……+1/19-1/20)/[1/(11×20)+1/(12×19)+……+1/(15×1
1*1/2+1/2*1/3+1/3*1/4……1/19*1/20
1/1*2*3+1/2*3*4+……+1/18*19*20
1+2+3+4+5+…+18+19+20=
1/1*2*3+1/2*3*4+1/3*4*5+……+20*21*22=?
(1-1/2)*(1-1/3)*(1-1/4)*(1-1/5)*(1-1/6)*…*(1-1/19)*(1-1/20)=
1-1/2+1/3-1/4+1/5-1/6+1/7-1/8+1/9……+1/19-1/20=?
计算:20-19+18-……-3+2-1=?
1/2*3*4+1/2*3*4+3*4*5+……1/18*19*20=