[Antironix]

You have a triangle like this


a b
----
| /
|/
c

To calculate a-c you need to know b-c and use co-sinus.

Well, that's a little, i am tired right now, i will give you what cos, sin and tan does.

Sin = Looking from the corner b, oposide(a-c) : diagonal side(b-c)
Cos = Opposide : diagonal side
Tan = Opposide : the line connected with the start corner. Like if you have the "start" corner c, the "line" is a-c

diagonal side is the side what doesn't have a 90 degree corner.

I know, it's a little bit messy, but i hope you understand some more.