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1. 等差数列求和// 首项a1，末项an，项数nll arithmetic_sum(ll a1, ll an, ll n) {return n * (a1 + an) / 2;}// 首项a1，公差d，项数nll arithmetic_sum(ll a1, ll d, ll n) {return n * a1 + n * (n - 1) / 2 …
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1. 等差数列求和// 首项a1，末项an，项数nll arithmetic_sum(ll a1, ll an, ll n) {return n * (a1 + an) / 2;}// 首项a1，公差d，项数nll arithmetic_sum(ll a1, ll d, ll n) {return n * a1 + n * (n - 1) / 2 …
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