Cosine of two theta is the same thing, cosine of two theta is the same thing as one minus two sine squared theta, which is just a difference of squares. We can rewrite that as, if this is a squared minus b squared, this is a plus b times a minus b. So I can just replace this with one plus square root of two sine theta times one minus square root of two sine theta. And now we have some nice canceling, or potential canceling that can occur. So we could say that cancels with that. And we could say that that is going to be equal, and let me do this in a new color, this is going to be equal to, in the numerator we just have one, in the denominator we just are left with one minus square root of two sine theta. And if we want these expressions to truly be equal, we would have to have them to have the same, if you view them as function definitions, as having the same domain.