# How to Use Trig Identities to Simplify an Expression

By Stefan RobertUsing trig identities to simplify and expression will require the use of a few key techniques, like sine and cosine. Use trig identities to simplify an expression with help from an expert in computers, with two degrees in both Computer Science and Applied Mathematics in this free video clip.

### Transcript

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.