Surface Area and Circumference

Working with Circumference & Area

Multiple Choice

What is the radius of the circle?

• 18

• 9

• 18 in.

• 9 in.

Multiple Choice

A circle has a radius of 3 cm. Find the area of the circle. Use 3.14 for pi.

• 28.26  $$cm^2$$  

• 9.42  $$cm^2$$  

• 18.84  $$cm^2$$  

Learning Target

• I can find the radius of a circle when given the circumference or area.

Find the Diameter

•  $C=2\pi r$  

•  $43.96=2\pi r$  

•  $43.96=\left(6.28\right)r$  

•  $\frac{43.96}{6.28}=r$  

• r = 7 units
  

• d = 14 units

Find the Radius

•  $A=\pi r^2$  

•  $113.04=\pi r^2$  

•  $\frac{113.04}{3.14}=r^2$  

•  $36=r^2$  

•  $6=r$  

Multiple Choice

What is the diameter of a circle with a circumference of 47.1 cm?

• 12 units

• 15 units

• 7.5 units