## Practice questions

- Based on this diagram,
find the circle's radius.

- Based on this diagram,
find the circumference of the circle.

## Answers and explanations

- The correct answer is
**Choice (B).**The formula for the area of a circle is

so create your own equation using the information you already have:

- The correct answer is
**Choice (C).**First, notice that the answer choices are in terms of pi. That means you won't have to multiply by 3.14.

The formula to find the circumference of a circle is

Because you know the diameter is 210 feet, the radius is 105 feet. Use what you know in the formula:

You can solve the problem more quickly using an alternate formula for the circumference,

where

*d*is the diameter. Mentally insert*d*= 210 in for*d*, and you get