Prove that if $n>1$, the sum of positive integers less than $n$ and coprime to $n$ is $(1/2)na(n)$ where…

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Question 12(iii) Could anyone explain this part of the question to me.

What i tried
co-prime means that the two integers a and b are said to be relatively prime, mutually prime, or coprime (also written co-prime) if the only positive integer (factor) that divides both of them is 1

Take th number $3$ for example, then the sum of integers less than $3$ and co-prime to $3$ is $2+1=3$, $2$ and $1$ are the two integers co-prime to $3$ which thus satisfies the formula $0.5*n*a(n)$ where $n=3$, $a(n)=2$. However im unsure of how to prove it. Could anyone explain. Thanks

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