So assuming you've had a go at it, let's work through it. So just like last time, we could write, let's just set y being equal to this. Y is equal to the inverse cosine of x, which means the same thing as saying that x is equal to the cosine of y. Now let's take the derivative of both sides with respect to x. On the left-hand side, you're just going to have a one. We're just going to have a one. And on the right-hand side, you're going to have the derivative of cosine y with respect to y, which is negative sine of y times the derivative of y with respect to x, which is dy dx.