Undue Obsession I

I was thinking about the Math question from last night and I can't think of why that solution works. This is how the text book's example does it, so I just helped my brother with that solution. I didn't want him to get all confused by showing him how to do it a different way.

It must be a shortcut that's been derived from some principle that I don't know about. I've turned Anonymous Posting back on, so that Boss-Friend can say something if he's so inclined =) =)

Boss-Friend is an Uber-Math guy. He's my Secondary Hero. =D My Primary Hero, though, is Father Man. He's the Lord High God of the Mathematical Universe.

Anyway... I'm trying to prove that I can do it the way that makes sense to me...