Recall the formula for the area and circumference.
Area of a circle: [tex]\pi \cdot r^{2}[/tex]
Circumference of a circle: [tex]2\pi \cdot r[/tex]
Now, we can make [tex]\pi[/tex] the subject in the circumference and substitute that into the area.
[tex]C = 2\pi r[/tex]
[tex]\frac{C}{2r} = \pi[/tex]
Substitute it in place of [tex]\pi[/tex].
[tex]A = \pi \cdot r^{2}[/tex]
[tex]A = \frac{C}{2r} \cdot r^{2}[/tex]
[tex]A = \frac{C \cdot r}{2}[/tex]
