Circles: radius, diameter, circumference and Pi | Geometry | Khan Academy
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A circle is at the foundation of geometry and how its parts relate to each other is both completely logical and a wonder.
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https://www.khanacademy.org/math/geometry/basic-geometry/circum_area_circles/e/radius_diameter_and_circumference?utm_source=YT&utm_medium=Desc&utm_campaign=Geometry
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https://www.khanacademy.org/math/geometry/basic-geometry/heron_formula_tutorial/v/part-2-of-the-proof-of-heron-s-formula?utm_source=YT&utm_medium=Desc&utm_campaign=Geometry
Geometry on Khan Academy: We are surrounded by space. And that space contains lots of things. And these things have shapes. In geometry we are concerned with the nature of these shapes, how we define them, and what they teach us about the world at large--from math to architecture to biology to astronomy (and everything in between). Learning geometry is about more than just taking your medicine ("It's good for you!"), it's at the core of everything that exists--including you. Having said all that, some of the specific topics we'll cover include angles, intersecting lines, right triangles, perimeter, area, volume, circles, triangles, quadrilaterals, analytic geometry, and geometric constructions. Wow. That's a lot. To summarize: it's difficult to imagine any area of math that is more widely used than geometry.
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