Vidiyal is going to talk about how to find the arc length and assert their heart. First thing we have to realize is we're given some radius and some angle, and we're trying to find the arc length. That is part of a circle, so instead of development of formula, we can remember how to do it in the future. So if I say find the arc length for the following case, for an angle of 90 degrees, we know that an angle for a circle needs to be above 90 degrees. So we know that this case has an acute angle. But we can also write this as a fraction of 180 degrees. Well, we know how to answer this pretty quickly just using our logic. We know the distance all the way around the circle, it's called the circumference. We know that's equal to 2 pi r or pi d. We know we have a radius of three, so at this time we would have a diameter of six inches. So the circumference of the entire circle is 6 pi inches. Well, it's obvious that we only have half of the surface, so it's pretty easy to take half of 6 pi inches in this case. So the arc length here will be 3 pi inches. The real question is how do we find that? That is something that makes sense when it's not so obvious. What you have to pick up here is that you know it's half because of how many degrees out of how many. A circle always has 360 degrees, it's always proportional. In this case, you have to recognize that the degrees are proportional to the arc length. Anytime you're asked to do this, you can quickly set up a ratio. So if...