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Prove that, if $x$ and $y$ are both odd positive integers, then $x^2 + y^2$ is even but not divisible by $4$.
Given:
$x$ and $y$ are both odd positive integers.
To do:
We have to prove that $x^2 + y^2$ is even but not divisible by $4$.
Solution:
Let $x=2 m+1$ and $y=2 m+3$ are odd positive integers, for every positive integer $m$.
This implies,
$x^{2}+y^{2}=(2 m+1)^{2}+(2 m+3)^{2}$
$=4 m^{2}+1+4 m+4 m^{2}+12 m+9$ (Since $(a+b)^{2}=a^{2}+2 a b+b^{2}$)
$=8 m^{2}+16 m+10$
$=8m^{2}+16m+8+2$
$=4(2m^2+4m+2)+2$
Hence, $x^{2}+y^{2}$ is even for every positive integer $m$ but not divisible by 4.
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