Area of a Circle

Area of a circle  =  \( \scriptsize \pi r^2 \)

We can also find the area given the diameter

the radius of a circle is half of its diameter

r = \( \frac{d}{2} \)

We can substitute this into Area of a circle   =  \( \scriptsize \pi r^2 \)

Therefore Area = \(\scriptsize \pi \: \times \: \normalsize \frac{d}{2} \scriptsize \: \times \: \normalsize \frac{d}{2} \\ = \frac{\pi d^2}{4} \)

To calculate the Area of a Semi-circle, divide the area of a circle by 2 (as two semi-circles make a circle). We get, \( \frac{1}{2} \; \times \; \scriptsize \pi r^2 \)