### Home > MC2 > Chapter 5 > Lesson 5.3.4 > Problem5-125

5-125.

The circle shown at right has a diameter of $20$ cm.

1. What measurement do you need to find in order to calculate the area inside the circle?

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?

2. Find the area inside the circle using the formula $A = r^2 π$. Write your answer as a product of $r^2$ and $π$, and as an approximation using $π ≈ 3.14$.

Diameter $= 2$ (radius)
Solve for $r$.

Substitute the newly calculated radius into the area formula.
$20 = 2 r$
$A = (10)^2 π$

$314$ cm$^{2}$