title: Problem 625
date: 2018/04/21 16:00:00
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# [Problem 625](https://projecteuler.net/problem=625)
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[Xem đề gốc (tiếng Anh)](https://projecteuler.net/problem=625)

## **Tổng GCD**

$G(N) = \sum_{j=1}^{N} \sum_{i=1}^{j} \gcd(i,j)$. Tính $G(10^{11})$.

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