I've just been looking at some algebra called intergrals.

I understand this fully but in the next step when he simplifies it a little he turns the into

I don't understand why he put a 1 there in place of the . Where did the 1 come from? Also one other question I have which I have is:

I have a basic understand of sin, cos and tan but what exactly do the represent when used in equations?

Can someone just explain what these equation actually means? Thanks