等比数列求和:x^0 + x^1 + ... + x^k
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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)
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)
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