A = pi * r2, so by dividing both sides of the equation by pi then taking the square root of both sides, you get...
r = (A/pi)½
(r equals the square root of the quotient of A divided by pi.)
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Diameter is half of the radius. Times it by 2.
circumference = diameter x pi area = radius squared times pi divide circ with pi gives you the diameter diameter/2 gives you radius radius times itself times pi gives you the area get the units right and you're done.
Circumference = 2*pi*radius Radius = Circumference/(2*pi)
The area increases as the square of the radius (or diameter). So if you double the radius you * 4 (quadruple) the area. Treble the radius, you *9 the area.
Multiply the radius by itself, then multiply the result by Pi.