Warm Up Problem of the Day Lesson Presentation Lesson Quizzes

Warm Up Evaluate. 1. 6  42 2. 5.6  23 Simplify each expression. 3. 6 + n – 4 4. 5(n + 1) 96 44.8 n + 2 5n + 5

Problem of the Day What is the area of a right triangle with legs measuring 31 in. and 21 in.? 325.5 in2 3

Learn to find the area of a circle.

Additional Example 1A: Estimating the Area of a Circle Estimate the area of the circle. Use 3 to approximate . Write the formula for area. A = r2 Replace  with 3 and r with 20. A ≈ 3 • 202 Use the order of operations. A ≈ 3 • 400 A ≈ 1,200 m2 Multiply.

Additional Example 1B: Estimating the Area of a Circle Estimate the area of the circle. Use 3 to approximate . A = r2 Write the formula for area. r = d ÷ 2 The length of the radius is half the length of the diameter. r = 28 ÷ 2 r = 14 Divide. A ≈ 3 • 142 Replace  with 3 and r with 14. A ≈ 3 • 196 Use the order of operations. A ≈ 588 m2 Multiply.

Estimate the area of the circle. Use 3 to approximate . Check It Out: Example 1A Estimate the area of the circle. Use 3 to approximate . Write the formula for area. 24.8 m A = r2 Replace  with 3 and r with 25. A ≈ 3 • 252 Use the order of operations. A ≈ 3 • 625 A ≈ 1,875 m2 Multiply.

Estimate the area of the circle. Use 3 to approximate . Check It Out: Example 1B Estimate the area of the circle. Use 3 to approximate . A = r2 Write the formula for area. 20 m r = d ÷ 2 The length of the radius is half the length of the diameter. r = 20 ÷ 2 r = 10 Divide. A ≈ 3 • 102 Replace  with 3 and r with 10. A ≈ 3 • 100 Use the order of operations. A ≈ 300 m2 Multiply.

Additional Example 2A: Using the Formula for the Area of a Circle Find the area of the circle. Use for . 22 7 A = r2 Write the formula to find the area. r = d ÷ 2 The length of the diameter is twice the length of the radius. r = 8 ÷ 2 = 4 A  • (4)2 22 7 Replace  with and r with 4. 22 7 __ A  • 16  22 7 __ 352 7 Use the order of operations. 2 7 A  50 ft2 Divide.

Additional Example 2B: Using the Formula for the Area of a Circle Find the area of the circle. Use for . 22 7 A = r2 Write the formula to find the area. A  • 72 22 7 Replace  with and r with 7. 22 7 __ A  • 49 22 7 __ 7 Use the GCF to simplify. 1 A  154 cm2 Multiply.

Find the area of the circle. Use for . 22 7 Check It Out: Example 2A Find the area of the circle. Use for . 22 7 28 cm A = r2 Write the formula to find the area. r = d ÷ 2 The length of the diameter is twice the length of the radius. r = 28 ÷ 2 = 14 A  • 142 22 7 Replace  with and r with 14. 22 7 __ A  • 196 22 7 __ 28 Use the GCF to simplify. 1 A  616 cm2 Multiply.

Find the area of the circle. Use for . 22 7 Check It Out: Example 2B Find the area of the circle. Use for . 22 7 7 cm A = r2 Write the formula to find the area. A  • 72 22 7 Replace  with and r with 7. 22 7 __ A  • 49 22 7 __ 7 Use the GCF to simplify. 1 A  154 cm2 Multiply.

Additional Example 3: Application A drum head has a radius of 18 in. Find the area of the material covering the drum head. use 3.14 for . A = r2 Write the formula for area. 18 in. A  3.14 • 182 Replace  with 3.14 and r with 18. A  3.14 • 324 Use the order of operations. A  1,017.36 in.2 Multiply.

Additional Example 3 Continued A drum head has a radius of 18 in. Find the area of the material covering the drum head. use 3.14 for . Check 18 in. Use 3 as an estimate for. The area, r2, is approximately 3 • 182 = 3 • 324 = 972, so the answer is reasonable.

A cymbal has a diameter of 18 in. Find its area. Use 3.14 for . Check It Out: Example 3 A cymbal has a diameter of 18 in. Find its area. Use 3.14 for . A = r2 Write the formula for area. r = d ÷ 2 The length of the radius is half the length of the diameter. 18 m r = 18 ÷ 2 r = 9 Divide. A  3.14 • 92 Replace  with 3.14 and r with 9. A  3.14 • 81 Use the order of operations. A  254.34 m2 Multiply.

Check It Out: Example 3 Continued A drum head has a radius of 18 in. Find its area. Use 3.14 for . Check 18 m Use 3 as an estimate for. The area, r2, is approximately 3 • 92 = 3 • 81 = 243, so the answer is reasonable.

Lesson Quizzes Standard Lesson Quiz Lesson Quiz for Student Response Systems

Estimate the area of each circle. 1. 2. Lesson Quiz: Part I Estimate the area of each circle. 1. 2. 1,200 yd2 27 km2 19

Find the area of each circle. Use for . 22 3. 4. 7 Lesson Quiz: Part II Find the area of each circle. Use for . 22 3. 4. 7 1.54 m2 4.53 cm2 5. A coater has a diameter of 6 inches. Find the area of the largest cup the coaster can hold. Use 3.14 for . 28.26 in2 20

Lesson Quiz for Student Response Systems 1. Estimate the area of the circle. Use 3 for . A. 48 km2 B. 46 km2 C. 44 km2 D. 42 km2 4 km

Lesson Quiz for Student Response Systems 2. Estimate the area of the circle. Use 3 for . A. 772 yd2 B. 872 yd2 C. 972 yd2 D. 992 yd2 36 yd

Lesson Quiz for Student Response Systems 3. Estimate the area of the circle. Use 3 for . A. 36.75 cm2 B. 21 cm2 C. 10.5 cm2 D. 9.19 cm2 3.5 cm

Lesson Quiz for Student Response Systems 4. A circular table has a diameter of 8 feet. Identify the area of the table cloth needed to cover the table. Use 3.14 for . A. 25.12 ft2 B. 37.68 ft2 C. 40.82 ft2 D. 50.24 ft2