Area of a Circle

The area of the circle is equal with pi times the square of the radius.

r = radius of the circle
d = diameter of the circle

d = 2r

$$Area(Circle) = \pi r^2 = \frac{\pi d^2}{2}$$

In this figure we have:
r = AB = AC
d = BC

so:

$$Area(Circle) = \pi AB^2 = \frac{\pi BC^2}{2}$$
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