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A closed curve every point on which is a fixed distance (the radius) from a point (the centre) within the curve. The diameter is a line that joins two points on the circumference and passes through the centre: the diameter is twice the radius (r). The circumference of a circle is equal to 2πr; the area is πr2, where π is a constant with the value 3.141 592. In analytical geometry the equation of a circle, centred at the origin, is x2 + y2 = r2.

A circle.

Subjects: Mathematics — Physics.

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