Question

Calculate the area of a circle with a diameter of 6cm

Answer 1

A≈28.27cm² is the area of the circle with diameter 6cm
as
 Area of a circle = pi * r² 

Since the diameter is twice the radius, the radius is 6cm/2 which is 3cm. 

pi * 3 * 3 

= 28.2743339cm²

Answer 2

Area of circle = πd²/4,    where d = diameter
 
                      = π*6*6/4 = 9π ≈ 9 *3.1416 ≈ 28.2744 cm²

Area ≈ 28.27 cm²

Hope this explains it.