Question

If the area of a circle is 121π square units, what is its diameter?

Answer 1

1 )  A = 121 π 
A = r² π 
r² π = 121 π
r = √ 121 = 11
The diameter: d = 2 r = 2 * 11 = 22 units
2 ) C = 20 π
C = 2 r π
2 r π = 20 π
r = 20 : 2 = 10 units
A = r² π = 10² π = 100 π square units.

Answer 2

Area of a circle is defined as πr2 and the diameter as 2r. Plugin the value and you can solve problem c. As for problem d, circumference is 2πr, plugin the value and solve the equation.