Exercise 2.3.12 (Number of positive integers $n\leq 7200$ prime to $3600$.)

Proof. Since $k\wedge 3600=1$ is equivalent to $(3600+k)\wedge 3600=1$, there are as many integers prime with $3600$ in $[[1,3600]]$ than in $[[3601,7200]]$. Therefore, the number $N_2$ of positive integers $n\le 7200$ that are prime to $3600$ is

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