What I encourage you in this video is to pause it and try to do the same type of proof for the derivative of the inverse cosine of x. So what is, so our goal here is to figure out, I want the derivative with respect to x of the inverse cosine of x. What is this going to be equal to? 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.