How do you find the area of the circle if you only know the circumference? For Example A circle has a circumference of cm. What is the area of the circle?

In this lesson you will learn how to find the area of a circle by using the measure of the circumference.

Let’s Review Area of a Circle = π r 2 Circumference of a Circle= π d or 2 π r

A Common Mistake Divide the diameter by 2 to find the radius. Find the area of a circle with a diameter of 6 cm. Wrong A = π (6cm) 2 A = 36 π cm 2 Correct A = π (3cm) 2 A = 9 π cm 2

Let’s Review Core Lesson A playground in the shape of a circle has a circumference of 18 π yards. What is the area of the playground? C = 18 π yds. C = π d D = 18 yds 18 yds

Let’s Review Core Lesson Area = π (9 yds) 2 Diameter = 18 yards Radius = 9 yards Area = 81 π yds 2 18 yds 9 yds

Let’s Review Core Lesson A circle has an approximate circumference of cm. If 3.14 was used for pi, what is the area of the circle? C = π d 37.68cm = 3.14d 37.68cm = 3.14d d = 12 cm

Let’s Review Core Lesson d = 12 cm r = 6 cm A = 3.14(6cm) 2 A = 3.14(36cm 2 ) A = cm 2 A circle has an approximate circumference of cm. If 3.14 was used for pi, what is the area of the circle?

In this lesson you have learned how to find the area of a circle by using the measure of the circumference.

Let’s Review Guided Practice The circumference of a circle is 10 π. What is the area of the circle?

Let’s Review Extension Activities The circumference of the Earth is approximately 25,000 miles. If you were to dig a hole to the center of the Earth, how long would the tunnel be? Explain how you found your answer.

Let’s Review Extension Activities The Redwood trees in California are said to have trunks so large that a car could fit in them. Estimate the circumference of the Redwood tree. Use your estimate to then find the area of the trunk if the tree was cut down. Explain your reasoning.

Let’s Review Quick Quiz The circumference of a circle is ft ². If 3.14 was used to approximate pi, what is the area of the circle? A circle has a circumference of 9 π meters. Find the area of the circle.