Area of a circle = pi*radius2
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The formula depends on the information available to you.
The radius of a circle is the distance from the center to any point on the circle. The area is the space within the circle. The formula to find the area is πr2. r stands for the radius of the circle. If you want to find the radius, you can work backwards from the area or the circumference, which is the perimeter of the circle. The formula for circumference is 2πr.
-- Take the formula for the area of the circle in terms of the radius . . . A = (pi) R2-- Solve that formula for 'R'. You'll then have a formula for the radius in terms of the area,which is exactly what you're looking for.
In order to find the diameter of a circle if you have the area is to rearrange the formula for the area of a circle to put diameter as the unknown, like so: PIE*d = Area of a Circle d = (Area of a Circle) / PIE Therefore if the area of your circle is 40cm, 40/PIE = 12.73cm and that's the diameter.
area of sector = (angle at centre*area of circle)/360