等差
1+2+3+...+n = n(1+n)/2
1+2^2+3^2+4^2+..+n^2 = n(n+1)(2n+1)/6
1+2^3+3^3+4^3+..+n^3 = (n^2)(n+1)^2 / 4
sn = n(a1 + an)/2
等比
sn = (a1 - qan)/(1 - q)