### Home > INT2 > Chapter 8 > Lesson 8.4.1 > Problem8-81

8-81.

Use your findings from problem 8-79 to answer the questions below. Recall that the diameter of a circle is a segment with endpoints on the circle that passes through the center. The length of the segment is also called the diameter.

1. What is the area of a circle with radius $10$ units?

For a circle, area $= r^2 · π$.

area $= 10^2·π = 100π$ units$^2$

2. What is the circumference of a circle with diameter $7$ units?

For a circle, circumference $= d·π$.

3. If the area of a circle is $121π$ square units, what is its diameter?

Use the formula for area of circle and solve for the radius.

$r = 11$ units
What is the diameter?

4. If the circumference of a circle is $20π$ units, what is its area?

Use the circumference formula and solve for the diameter.