Question: How do you find the area of a circle?
The area of a circle is given by,
A = piR^2
where R is the radius of the circle and pi (the greek letter for p) is a constant and equals 3.14159 (approx)
Your example:
A circle with a diameter of 21 cm.
How would you find the area of that circle?
Diameter = 21 cm, so Radius = (1/2) Diameter = 10.5 cm
A = pi(10.5)^2 = pi*110.25 = 3.14159*110.25 = 346.36
Answer: Area = 346.36 cm^2