1-2/1*(1+2)-3/(1+2)*(1+2+3)-4/(1+2+3)*(1+2+3+4)-……-20/(1+2+3
1-2/1*(1+2)-3/(1+2)*(1+2+3)-4/(1+2+3)*(1+2+3+4)-……-20/(1+2+3
1-1*(1+2)分之2-(1+2)*(1+2+3)分之3-(1+2+3)*(1+2+3+4)分之4-……-(1+2+…
(-1*2/1)+(-1/2*1/3)+(-1/3*1/4)+…+(-1/2012*1/2013)
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
求和:1/1×2+1/2×3+1/3×4+……+1/n(n+1)
1/2*1+1*2*3+1/3*4+………+1/99*100
| 1/3 - 1/2 | + | 1/4 - 1/3 | + | 1/5 - 1/4 | …… + | 1/2009
计算(1-1/2^2)(1-1/3^2)(1-1/4^2)……(1-1/50^2)
(1-1/2^2)(1-1/3^2)(1-1/4^2)……(1-1/2010^2)
巧算:(1/1+2)+(1/1+2+3)+…+(1/1+2+3+…+20)
计算1/1×3+1/2×4+1/3×5……1/18×20