That is the same thing as saying that the tangent of y is equal to x. All I've done is, you can kind of think of it as, I've just taken the tangent of both sides right over here. Now we can take the derivative of both sides with respect to x. On the left-hand side, we can just apply the chain rule. Derivative of tangent of y with respect to y is going to be secant squared of y, which is the same thing as 1 over cosine of y squared. I like to write it this way. It keeps it a little bit simpler, at least in my brain.