it uses to find out thearea and circumference of circle.
diameter=2*radius
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diameter = 2rarea = pi*r^2
Circumference = pi*diameter or 2*pi*radius
double the radius
The question is in really bad english, but I'm guessing your meant: 'How do I work out the diameter using pi?' Diameter= Radius2 You don't need to use Pi to work out the diameter. Unless you have a question that doesn't show you the radius. Then you do Area divided by Pi which equals the Diameter.
Divide the circumference by 3.14159265 (3.14 for rough approximation) to find the diameter of a circle.