Area of the circle can be written as:
[tex]Area= \pi r^{2} [/tex]
Diameter is twice the radius, which means
Diameter = d = 2r
or
r = d/2
Using this value of r, we can write the equation for Area as:
[tex]Area= \pi ( \frac{d}{2})^{2} \\ \\
Area= \frac{ \pi d^{2} }{4} [/tex]
