en.wikipedia.org/wiki/Area_of_a_circle

In geometry, the area enclosed by a circle of radius r is πr2. Here the Greek letter π represents a constant, approximately equal to 3.14159, which is equal to the ratio of the circumference of any circle to its diameter. One method of deriving this formula, which originated with Archimedes, ...

www.mathgoodies.com/lessons/vol2/circle_area.html

Area of a circle is presented in this interactive lesson from Math Goodies. ... In the last lesson, we learned that the formula for circumference of a circle is: C ...

www.mathwarehouse.com/geometry/circle/area-of-circle.php

Area of a circle, formula for area explained with examples, pictures, interactive examples and quiz.

www.khanacademy.org/math/basic-geo/basic-geo-area-and-perimeter/area-circumference-circle/v/area-of-a-circle

The area of a circle is pi times the radius squared (A = π r²). Learn how to use this formula to find the area of a circle when given the diameter.

www.virtualnerd.com/pre-algebra/perimeter-area-volume/circles/circle-sector-area-examples/circle-area-from-diameter

Note: If you have the diameter of a circle, you can use it to find the area of that circle. Just plug that value into the formula for the area of a circle and solve.

www.mathsisfun.com/geometry/circle-area.html

Area of a Circle. Calculator Enter the radius, diameter, circumference or area of a Circle to find the other three. The calculations are done "live":.

www.virtualnerd.com/pre-algebra/perimeter-area-volume/circles/circle-sector-area-examples/circle-area-formula

Virtual Nerd's patent-pending tutorial system provides in-context information, hints, and links to supporting tutorials, synchronized with videos, each 3 to 7 ...

www.coolmath.com/reference/circles-trigonometry

Definitions and formulas for the arc and the arc length of a circle, sector and the area of the sector of a circle, the unit circle, the angles on the unit circle in ...

www.varsitytutors.com/hotmath/hotmath_help/topics/area-of-a-circle

A circle is the set of all points in a plane at a given distance (called the radius ) ... Use the formula for area of a circle, A=πr2 , where r is the radius of the circle.