 know what circles look like. So in this video we're going to discover the equation of a circle thanks to how they look on a graph. So let's have a look at three different circles all with their center at the origin. What do you notice about the equations and the radius of each circle? I'm sure you're spotted that the equals number is always the radius squared. So 5 squared, 7 squared and 9 squared. So the equations are always x squared plus y squared equals the radius squared. What if the center isn't at the origin? Have a look at these three circles. I'm sure you're all spotted that the coordinates of the center point go in the brackets with the x and y remembering to change the sign. The x coordinate goes in the bracket with the x and the y coordinate goes in the bracket with the y. Center at x is 2 means the bracket is minus 2 and the center at y is minus 3 means the bracket is plus 3. That's all there is to it. Here are some questions for you to do. Pause the video, work out the answers and click play when you're ready. Did you get them right? So there we have the equation of a circle. You just need the center point and the radius. We'll have a look at how to find the equation of a tangent to a circle in another video.