Question for my brain and a link

I just spent half an hour starring at a piece of paper, knowing that there is a way to calculate the angles in a right-angled triangle when knowing how long the sides are, but what exactly this way was I just couldn't remember.
Asking my parents resulted in one of those 'Argh, of course' moments.

Trigonometric functions -- the one thing out of maths class, that leave my head as soon as I turn around.
When someone mentions them, I have no problems to remember how sine or cosine are defined and use them in calculations, but I need this starting point.

Dear brain,
could you please explain why this particular blind spot? Any special reason? I would have no problems forgeting about statistics, why geometry?

And I spend too much time reading
Internet Bumper Stickers.