### Home > INT2 > Chapter 8 > Lesson 8.4.1 > Problem 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. Homework Help ✎What is the area of a circle with radius 10 units?

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

If the area of a circle is 121

*π*square units, what is its diameter?If the circumference of a circle is 20

*π*units, what is its area?

For a circle, area = *r*^{2} · *π*.

area = 10^{2}·*π* = 100*π* units^{2}

For a circle, circumference = *d*·*π*.

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

*r* = 11 units

What is the diameter?

Use the circumference formula and solve for the diameter.

What is the radius? Use the radius to calculate the area.