GED Math Extra Prep: Circumference and Area of a Circle - dummies

# GED Math Extra Prep: Circumference and Area of a Circle

The GED Math test will probably contain questions that ask you to work with the circumference and area formulas for a circle. As you’ll see in the following practice questions, some of these problems will be a little more involved than simply plugging in numbers.

## Practice Questions

A large circular pizza has an 18-inch diameter and is cut into 8 equal slices.

1. What is the perimeter of one slice of the pizza?
2. What is the radius (in cm) of a circle whose circumference happens to equal its area (in cm2)?
A. 2 cm
B. 3 cm
C. 5 cm
D. 10 cm

1. The correct answer is C.

The equation for the circumference of a circle is

Substituting the value of the diameter into the equation gives you

The perimeter of the slice is the distance around the edge of the slice, which equals two radii (2 times 9 inches) plus the length of the curved edge. There are eight pieces of pizza, so the length of the curved edge of the slice represents one-eighth of the entire circumference:

The perimeter of the slice equals

so the correct answer is Choice (C).

2. The correct answer is A.

The equation for the area of a circle is

and the equation for the circumference of a circle is

Because the circumference of this particular circle happens to equal its area, you can set the equations equal to each other and solve for r. Dividing both sides of the equation by

gives you

Therefore, Choice (A) is the correct answer.