 In this video we're going to look at the exterior angles of polygons. Firstly what they are and secondly the sum of exterior angles. We looked at interior angles in another video. We looked at the sum of interior angles and the size of one interior angle. If you're unsure about them you may want to watch that video first. So let's have a look at exterior angles. They're a little strange. Exterior angles make a straight line with the interior angle. They aren't the whole outside angle but just the part that makes a straight line. What's the exterior angle of a regular nonagon? This is where we need to know about interior angles first. So you may want to watch this video. Start by finding the sum of interior angles. Nine sides means seven triangles so seven times 180 means there are 1,260 degrees in a nonagon. So one interior angle is 1,260 divided by nine sides which is 140 degrees. 180 take away 140 gives us the exterior angle of 40 degrees. Simple! Here's one for you to do. Pause the video, find the angle and click play when you're ready. Did you get it as 50 degrees? If we take all of the exterior angles and pull them together they will always make 360 degrees. This is true for any polygon the exterior angles will always add up to 360 degrees. This is actually really useful. We can use it to work out how many sides a regular polygon has. We're given the exterior angle of 45 degrees. As the exterior angles always add up to 360 we know that 360 divided by the number of angles will equal the exterior angle. Or simple rearranging 360 divided by the exterior angle will give the number of sides. So 360 divided by 45 means this is an octagon. Here's one for you to do. How many sides does this regular polygon have? Pause the video, whack it out and click play when you're ready. Did you get it right? It's a 15 sided polygon. So there we have finding the interior and exterior angles of polygons. For the sum of interior angles you subtract 2 from the number of sides and multiply it by 180. You can always fit two less triangles in. Exterior angles always make 180 degrees with the interior angle. And exterior angles always add up to 360 degrees. Combining knowledge of all of these facts we can work out any interior or exterior angles. Just make sure if it's a regular or irregular polygon first. Like and share our videos with your friends. If you have any questions comment below and I'll help you out.