pi is the ratio of a circle's diameter (the distance across it) to its circumference (the distance around it).
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Pi is the number you have to use to solve many circle equations. If you want to find the area of a circle, use radius squared times pi. For the circumference, use diameter times pi.
Circumference of a circle = pi*diameter or 2*pi*radius
To find the area of half a circle, use the formula for a circle pi*r^2 divided by 2. pi = 3.14, r = radius (half the distance across the circle)
circumference of a circle = diameter*pi or 2*pi*radius
2 * pi * r