Prove: $\sum_{k<n, (k,n)=1} k= \frac{1}{2}n \varphi (n)$

Prove: $\sum_{k<n, (k,n)=1}k = \frac{1}{2}n \varphi (n)$

I have had strep throat and missed the lecture discussing properties of the Euler function. Any help in solving this is appreciated. Thank you!

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