How do Icalculate the area of a circle from its diameter?

\(A_{circle}=\pi r^2\\ d_{circle}=2r\)

 

\(d_{circle}=2r\\ r=\frac d2\\ A_{circle}=\pi \times (\frac{d}{2})^2\\ A_{circle}=\pi \times \frac{d^2}{4} \)

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