# Area of

Apuntes

# Area of a Circle ## Key Vocabulary Area: the number of square units inside the circle. Radius: the distance from the center of the circle to anywhere on the circumference of the circle Diameter: the distance from one point on the circle, through the center, to another point on the circle π: pronounced "pl." It is the ratio of the circle's circumference to the circle's diameter. π is about 3.14  Did You Notice Diameter = 2 x Radius $$d = 2r$$ ## Area of a Circle Formula: $$A = \pi r^2$$ Finding Area Example 1:  $$A = \pi r^2$$ $$A = \pi \cdot 6^2$$ $$A = 36 \cdot \pi$$ $$A = 36 \cdot 3.14$$ $$A = 113.04 \text{ cm}^2$$ Finding Area Example 2:  $$d = 2r$$ $$18 = 2r$$ $$9 = r$$ $$A = \pi r^2$$ $$A = \pi \cdot 9^2$$ $$A = 81 \cdot \pi$$ $$A = 81 \cdot 3.14$$ $$A = 254.34 \text{ in}^2$$ Going "Backward" Example: The area of a circle is 200.96 mm². Find the radius of the circle (use 3.14 for pi). $$A = \pi r^2$$ $$\frac{200.96}{3.14} = \frac{3.14r^2}{3.14}$$ $$64 = r^2$$ $$\sqrt{64} = \sqrt{r^2}$$ $$8 = r$$ The radius of the circle is 8 mm. © Classroom 127 A circle is a colle...