Area and Circumference (Circles)

$$A=\frac{1}{2}\times \text{diameter} \times \text{radius}$$ or $$A=\text{π}r^2$$ .

$$C=2\text{π}r$$ or $$C=\text{π}d$$ .

The distance around a circle.

The distance from the center of the circle to any point on its circumference.

The distance across the circle through its center, equal to twice the radius.

$$A=\text{π}(5^2)=25\text{π} \text{ square inches}$$ .

$$C=\text{π}(10)=10\text{π} \text{ inches}$$ .

The diameter is twice the radius: $$d=2r$$ .

Multiply the radius by 2: $$d=2r$$ .

$$A=\text{π}(3^2)=9\text{π} \text{ square inches}$$ .