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Radius, diameter, center, and circumference--all are parts of a circle. Let's go through each and understand how they are defined. Created by Sal Khan and Monterey Institute for Technology and Education.
Video transcript
Draw a circle and label the radius, diameter, center and the circumference. Let me draw a circle, it won't be that well drawn of a circle but I think you get the idea so that is my circle. I'm going to label the center..over here. So I'll do the center I'll call it 'c'. So that is my center....and I'll draw an arrow there that is the center..of the circle and actually the circle itself is the set of all points that are a fixed distance away from that center and that fixed distance away, they're all from that center..that is the radius. So let me draw..the radius. So this distance right over here is the radius That is the radius and..that's going to be the same as this distance! ..which is the same as that distance! I can draw multiple radii. All of these are radii The distance between the center and any point on the circle now a diameter just goes straight across the circle going throooouugh the center from one side of the circle to the other side..going though the center and it's essentially two radii put toghether. So for example this would be a diameter.. that would be a diameter, you have one radii and another radii all on one line going from one side of the circle to another going through the center. So that is a diameter! ^_^........ that is a diameter. and I could have drawn it other ways, I could have drawn it like this that would be another diameter it would have the exact same length and finally, we have to think about the circumference, and the circumference is really just how far you have to go to go around the circle or if you put a string on that circle, how long would the string have to be? so what I'm tracing out in blue right now the length of what im tracing out is the circumference so....right over here..that is the circumference cir-cum-ference. Cir-cum-ference. And we're done!