



 
Hi Al, The radius $r$ of a circle is half the diameter $d$ and thus \[\pi \; r^2 = \pi \left(\frac{d}{2}\right)^2 = \pi \left(\frac{d}{2}\right)\times \left(\frac{d}{2}\right)=\pi \left(\frac{d \times d}{2 \times 2}\right) = \pi \left(\frac{d^2}{4}\right) = \frac{\pi}{4} d^2 .\] I hope this helps,  


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