On: A visual proof that a^2 – b^2 = (a + b)(a – b)

[konschubert]

The expression immediately follows from

a ( b+c) = ab + ac.

I agree it's important that students intuitively feel that

a ( b+c) = ab + ac

is true.

But once we are at that, why would we even bother to teach or "proof" all variations of that expression?