What is the difference between radius & diameter?
Diameter is the longest line that you can draw in a circle passing through the centre of the circle.
Radius is the line joining the centre of the circle to any point on the circumference.
Diameter is the ratio of the circumference of the circle to the irrational constant 'pi' (3.1415...). Radius is the half of the diameter.
Actually pi is defined to be the ratio of the circumference of the circle to the diameter of the circle. Therefore, you can solve for the diameter from that equation.