What is the area of a circle with a diameter of 8?

Commit to memory

A = pi r^2

C = 2 pi r or pi d

NB 2r = d

This is elementary school maths.

Hence if d = 2r 8 , then r = 8/2 = 4

Hence

A = pi 4^2

A = 16 pi

pi is a constant at 3.141592....

A = 16*3.41592.... = 50.26554.... units^2