How can you find the circumference of a circle if you only know the area? For Example A circle has an area 16 cm 2. What is the circumference of the circle? In this lesson you will learn how to find the circumference of a circle by using the measure of area. Lets Review Area of a Circle = r 2 Circumference of a Circle= d or 2 r The reverse of squaring a number is finding the square root. 3 2 = 9 A Common Mistake Finding the square root of a number is not the same as dividing it by 2. Wrong Correct Lets Review Core Lesson Bill has a circular garden that he wants to put a fence around. He knows that the area of a garden is 16 yds 2. How much fencing does Bill need to go around the circumference of the garden? A = 16 yds 2 A = r 2 C = 2 r r 2 = 16 yds 2 r = 4 yds 4 yds C = 2 (4 yds) C = 8 yds Lets Review Core Lesson A circle has an approximate area of ft 2. If 3.14 was used for pi, what is the circumference of the circle? A = r ft 2 = 3.14r ft 2 = r 2 3 ft = r C = 2 r C = 2(3.14)3 C = ft In this lesson you have learned how to find the circumference of a circle by using the measure of the area. Lets Review Guided Practice A circle has an area of 25 cm 2. What is the circumference of the circle? Lets Review Extension Activities The area of a circle is 16 meters 2. Stephanie concluded that the circumference of the circle would be 16 . She stated the radius would be 8 meters since the square root of 16 is 8. What mistake did Stephanie make. Write an explanation describing how to fix her mistake. Lets Review Extension Activities A circle inside of a square touches the square at each edge as shown below. If the circle has an area of 25 feet 2, find the area of the square. Lets Review Quick Quiz Find the circumference of the circle that has an area of 81 m 2. A circle has an area of cm 2. What is the circumference of the circle?