There are fully general and valid geometric proofs, eg http://mathandmultimedia.com/wp-content/uploads/2010/02/pyth...
But I don't see how it proves anything if $a$ were say doubled, and $b$ kept the same.
* For any real positive values $a$ and $b$
* let $T$ be the (unique) right-angled triangle with non-hypotenuse sides of length $a$ and $b$
* then $a$^2 = $b$^2 = $c$^2
* where $c$ is the (unique) length of the hypotenuse of $T$.
You could give me the instance
$a$ = 2, $b$ = 3
and it will prove it for this case. Then you could later double $a$ and give me the instance $a$ = 4, $b$ = 3
and it will prove it just as easily for this case.