maths is simple, and just so you know, you never assume anything in maths. but you'd already know that since you're so good at picking mathematical faults.
If math were so simple do you honestly think it would cause so many people stress? Hell, I wouldn't doubt it if someone committed suicide because of it. And for most people, i'm sure thats why they drop out because stuff like that gets over complicated and just isn't worth learning. Please, you assume plenty. You can't know something is correct (at first at least). You have to assume that is.