title: Problem 795
date: 2022/04/23 06:00:00
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# [Problem 795](https://projecteuler.net/problem=795)
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[Xem đề gốc (tiếng Anh)](https://projecteuler.net/problem=795)

## **Tổng GCD xen kẽ**

$f(n) = \sum_{k=1}^{n} (-1)^{k+1} \gcd(k, n-k)$. Tính $\sum_{n=2}^{10^{11}} f(n)$.

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