The reason why the tangent of y is interesting is because we already know that tangent of y is equal to x. If we can rewrite this using a little bit of trigonometric identities, then with tangent of y, we can substitute all the tangent of y's with an x. Let's see if we can do that. This seems a little bit tricky. To introduce a tangent of y, we'd want to have a sine divided by a cosine. That's what tangent is. This is just a straight-up cosine squared y.