4.4 Multiplying Decimals and Circumference of a Circle Learning Objectives: 1. Multiply decimals. 2. Multiply by powers of 10. 22
3. Find the circumference of a circle. (Do not use л ≈ 3.14 or
) 7
4. Solve applications involving multiplication of decimals and circumference. 5. Key Vocabulary: л (pi), perimeter, circumference, diameter, and radius. 1. Multiplication of Decimals Steps: 1. Multiply the two decimal numbers as if they were whole numbers. 2. The number of decimal digits in the product is the sum of the number of decimal digits in the factors. Example 1. Multiply. 2. 0.51 × 0.0045 1. 0.012 × 0.3 2. Multiplying by Power of 10 To multiply a decimal number by 10, 100, 1000, or a higher power of 10—is to move the decimal point as many places to the right as there are zeros in the power of 10 being multiplied. Example 2. Multiply. 1. 58.124 × 100 2. 0.048 × 10000 To multiply a decimal number by 0.1, 0.01, 0.001, 0.0001—is to move the decimal point as many places to the left the same number of places as there are decimal places in the power of 10 being multiplied. Example 3. Multiply. 1. 0.07 × 0.1 2. 2.908 × 0.001 3. Finding the circumference of a circle Circumference, C = 2πr (use when radius is given) radius
(use when diameter is given) or C = πd diameter
Example 4. Find the circumference of a circle with the given information. Leave answer in term of π. 1. Radius ‐ 7 feet 2. Diameter ‐ 16 inches 4. Solving Problems by Multiplying Decimals Example 5. Elaine is fertilizing her garden. She used 5.6 ounces of fertilizer per square yard. The garden measures 60.5 square yards. How much fertilizer does she need? Read: Section 4.5/4.6