## Video and Examples

Finding the area of circles using the formula π • r • r

The area of any shape is the number of square units inside that shape. In the figure to the left, the square has an area of 36 square units, because there are 36 units inside the square. If each unit square in the circle were counted (including all the partial squares), there would be 28.26 unit squares inside the circle, so the area of this circle would be 28.26 square units. However, it is easier to use the following formula:

$A=\pi~r^2$ also known as $A=\pi~\cdot~r\cdot~r$

where A represents the area, and r is the radius of the circle.

Finding area using π = 22/7

Finding area using π = 3.14

 In terms of π.

 Use this applet to observe how to find the area of any circle using either $3.14$ or $3\frac{1}{7}$ and when given the radius or the diameter. Move the blue and green points to create the circle of your choice. Then use the slider on the left to choose whether the radius or the diameter is given. The slider on the right selects which version of $\pi$ is being used.

# Self-check

Q1: What is the area of a circle with a radius of 10cm? (Use π = 3.14) [show answer]

Q2: What is the area of a circle with a radius of 7cm? (Use π = 3 1/7) [show answer]

Q3: What is the area of a circle with a radius of 3cm in terms of π? [show answer]