[Math] Prove that even + odd is odd.

elementary-number-theoryproof-writing

Prove that a even number + odd number = odd number

Let $x$ be the even number, let $y$ be the odd number.

From the definition of odd numbers, $y + 1$ is even.

Let:

$$x + y = z$$

Suppose $z$ is even.

$$x + y = z$$

It follows that:

$$x = x + y – y = z – y$$

But I cant think of any other contradiction here. The issue is with the assumption I suppose. Any help is appreciated!

Best Answer

An even number $n$ can always be written as $2k$. An odd number $m$ can always be written as $2h+1$.

Hence $n+m=2k+2h+1=2(h+k)+1$ which is odd.

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