If $x^2=?iso-8859-1?Q?=3Dy^2$=2C_prove_that_$x=3Dy$_or_$x=3D-y$_=96_math.stackex?=change.com

If $x^2=y^2$, prove that $x=y$ or $x=-y$ – math.stackexchange.com

I have a simple question here. I am trying to prove that, given $x^2=y^2$,
$x=y$ or $x=-y$. I know exactly why this is true; it's obvious. I'm just
unclear on the general format of a proof, as well as …