### Home > MC2 > Chapter 5 > Lesson 5.3.4 > Problem 5-125

5-125.

**The circle shown below has a diameter of 20 cm. Homework Help ✎**What measurement do you need to find in order to calculate the area inside the circle?

Find the area inside the circle using the formula

*A = r*. Write your answer as a product of^{2}π*r*^{2}and*π*, and as an approximation using*π ≈*3.14.

Recall the equation for area from the Math Notes box of Lesson 5.3.4.

A = *r*^{2} π

What measurement is missing in this equation?

Diameter = 2 (radius)

Solve for *r*.

Substitute the newly calculated radius into the area formula.

20 = 2 *r*

*A* = (10)^{2} π

314 cm^{2}