The circumference of a circle is given by:
[tex]\begin{gathered} C=2\pi r \\ where\colon \\ r=radius \\ or \\ Since \\ d=diameter=2r \\ C=\pi d \end{gathered}[/tex]And the area is:
[tex]\begin{gathered} A=\pi r^2 \\ or \\ A=\pi(\frac{d}{2})^2 \end{gathered}[/tex]