x of t, x of t, is equal to the hypotenuse, 20 meters, that's the length of the ladder, times the cosine, cosine of theta, and I could say the cosine of theta of t just to make it clear that this is a function of time. This comes straight out of trigonometry, actually our basic trigonometric function definitions. Now, why is this useful? Why do I think this is useful? Well, let's think about what happens when I take the derivative of both sides using the chain rule. On the left-hand side, I am going to have an x prime of t, and then that's going to be equal to, what do I end up on the right-hand side? Well, using the chain rule, first I'll take the derivative with respect to theta, and so that's just going to be negative 20 sine of theta of t, and then I need to multiply that times theta prime of t. So, what I could do is say, hey, look, at t sub zero, I know what x prime of t is, I could try to figure out what sine of theta of t is, and then I'll just solve for this right over there.