Step-by-step explanation:
Area formula is
[tex]\pi {r}^{2} [/tex]
and we know that
[tex]2r = d[/tex]
Isolate r.
[tex]r = \frac{d}{2} [/tex]
Plug d/2 for r.
[tex]\pi( \frac{d}{2} ) {}^{2} [/tex]
or
[tex]\pi( \frac{d}{4} )[/tex]
