Hi, my name is Stefan. Today, I'm going to be speaking on how to use trig identities to simplify an expression. So let's say we have the expression Y = cosine squared theta + sine squared theta + sine theta/cosine theta. Alright this looks a little long and messy but you know what, there's a trick identity that says put a mark here, sine squared + cosine squared = 1 and also there's another identity that sine theta divided by cosine theta is equivalent to the tangent of theta. So if we substitute these in, this looks much cleaner. So we can rewrite this as Y = sine squared + cosine squared, that's 1, + the tangent of theta, sine over cosine = our tangent. So there's easier and much simpler. And that's how you simplify.