What is a radius? The geometry of a circle What is a radius? The radius of a circle is the distance from the center point to the edge of the circle. It's the same distance anywhere on the circle, because the circle has radial symmetry. So it doesn't matter where you measure the radius on the circle, [...]
What is pi? The geometry of a circle What is pi? Pi is a number that is just a little bigger than 3.14. It is the number you get if you divide the circumference of any circle by its diameter. It's the same for all circles. You can approximate pi for yourself by taking some circular things like the tops [...]
Proof for the area of a circle: Divide a circle up into sections like an orange Euclid's proof We know it's true that A=πr-squared is the area of a circle, but how can we prove that this formula is always true for every possible circle? Here's how Euclid did it. Who was Euclid? Area of a circle More [...]
Area of an oval The area of an oval is the part of a plane that is inside its circumference. To figure out the area, you need to measure the height and width of the oval. Multiply half the height by half the width, and then multiply that by π (pi). You figure out the area of a circle the same [...]
The geometry of a circle The area of a circle is the flat surface inside its circumference. The area is always equal to the radius squared, multiplied by π (pi). Here's a proof that this is always true for any circle. Learn by doing: figure out the area of some circles around you. Radius Diameter Circumference of [...]
Greek mathematics: An Egyptian papyrus from about 100 AD which is a piece of one of Euclid's books Greek math: geometry not algebra Because people in ancient Greece had only very clumsy ways of writing down numbers, they didn't like algebra. Ancient Greek numbers What is algebra? All our ancient Greece articles They found it very hard [...]
Chinese numbers: number rods Chinese numbers People in China were using written numbers by about 1500 BC, in the Shang Dynasty. This is about two thousand years later than people began to write numbers in West Asia. It's more than a thousand years later than people began to write numbers in India. Nobody knows whether [...]
