MHB Prove Even Integer is Multiple of 4: Contradiction Approach

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Every even integer that is the square of an integer is a multiple of four. The proof begins by assuming n is even and a square, then setting up a contradiction by assuming n is not a multiple of four. It demonstrates that if n is even, it must be of the form n = 2k, leading to n^2 = 4k^2, confirming that n^2 is a multiple of four. The lemma states that if n^2 is even, then n must also be even, reinforcing the theorem. Thus, the conclusion is that every even integer square is indeed a multiple of four.
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Every even integer that is the square of an integer is a multiple of four.

Prove by Contritidiction.

Assume that n is even and n is square.

I am lost to do next.
 
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Cbarker1 said:
Every even integer that is the square of an integer is a multiple of four.

Prove by Contritidiction.

Assume that n is even and n is square.

I am lost to do next.

... and assume n is not a multiple of 4 (setting up the contradiction).

Then n must be divisible by 2 but not by 4...

What can we say about the number that was squared?
Can we tell if it's even or odd?
 
Lemma: if $n^2$ is even then n is even.
Proof by contradiction- Suppose n is not even. Then it is of the form n= 2k+ 1 for some integer k. Then $n^2= (2k+1)^2= 4k^2+ 4k+ 1= 2(2k^2+ 2k)+ 1$ so is odd, not even.

Theorem: if $n^2$ is even then $n^2$ is a multiple of 4.
Proof- by the lemma, since $n^2$ is even, n is even. That is, n= 2k for some integer, k. Then $n^2= (2k)^2= 4k^2$.
 
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