We know that the area of a circle in terms of π will be πr². However the area with respect to the diameter will be a different story. The first step here is to find a function relating the area and diameter of any circle --- ( 1 )
For any circle the diameter is 2 times the radius,
d = 2r
Therefore r = d / 2, which gives us the following formula through substitution.
A = π(d / 2)² = πd² / 4
Hence the area of a circle as the function of it's diameter is A = πd² / 4. You can also say f(d) = πd² / 4.
Now we can substitute " d " as 4, solving for the area ( A ) or f(4) --- ( 2 )
f(4) = π(4)² / 4 = 16π / 4 = 4π - This makes the area of circle present with a diameter of 4 inches, 4π.