How to prove these problems? (Operation on N)

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  1. If $m+n = m+k$, then $n=k$.
  2. If $mk = nk$ and $k\neq0$, then $m=n$.

I'm not sure what t do. Do I have to use math induction or not? (Base on Peano system)

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If you want to prove these sort of things in Peano, then using induction is probably a strong candidate for the first attempt.

To prove the on about addition, use induction on $m$. Note that this requires you to know that $x+y=y+x$ first, which you can also prove using induction. Similarly, the multiplication should be proved by induction on $k$.