等比数列求和：x^0 + x^1 + ... + x^k

等比数列求和：x^0 + x^1 + ... + x^k

令S表示x^0 + x^1 + ... + x^k的和：
S = 1 + x + x^2 + ... + x^k

两边同时乘以x：
S*x = x * (1 + x + x^2 + ... + x^k)
       = x + x^2 + ... + x^k + x^(k+1)

两边再同时加1：
S*x + 1 = 1 + x + x^2 + ... + x^k + x^(k+1)
            = (1 + x + x^2 + ... + x^k) + x^(k+1)
            = S + x^(k+1)

即：
S*x - S = x^(k+1) - 1
S*(x - 1) = x^(k+1) - 1

则：
S = (x^(k+1) - 1) / (x - 1)