## Algebra and Trigonometry 10th Edition

$A=\frac{1}{2}r^2\theta$
Area of a circle: $A=\pi r^2=\frac{1}{2}r^2(2\pi)$ The area of a sector of a circle is proportional to the central angle: $A=\frac{1}{2}r^2(2\pi)(\frac{\theta}{2\pi})=\frac{1}{2}r^2\theta$