Proving an equation involving 4 real numbers

[Jadehaan]

Homework Statement

This is the question followed my my attempt at the solution:

Just wondering if this looks right?
Thanks for any and all feedback,
Jim

 

[Tinyboss]

I'll let someone else check the proof, but I thought you might like to know that "...as desired. QED" is redundant. QED is an abbreviation for "quod erat demonstrandum", meaning "which was to be demonstrated".

The (hollow or solid) square symbol is more common nowadays, which I've heard is a pun on quod/quad, but I don't know if that's true.