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# Chapter 3, Section 3, Page 102 “Adding & Subtracting Decimals”

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Chapter 3, Section 3, Page 102 “Adding & Subtracting Decimals”

Remember In this class Only use a

Warm Up Estimate by rounding to the indicated place value. 1.70.27 + 15.36; ones 2. 84.37 – 21.82; tenths Estimate each product or quotient. 3. 27.25  8.7 4. 44.52 ÷ 3.27 85 62.6 270 15

MA.6.A.5.3 Estimate the results of computations with…decimals…and judge the reasonableness of the results. Also Review of MA.5.A.2.2 Sunshine State Standards

Estimating before you add or subtract will help you check whether your answer is reasonable. Helpful Hint

Carly Patterson’s Preliminary Scores EventPoints Floor exercise9.7 Balance beam9.7 Vault9.3 Uneven bars9.45 At the 2004 U.S. Gymnastics Championships, Carly Patterson and Courtney Kupets tied for the All-Around title. To find the total number of points, you add all of the scores.

Additional Example 1A: Sports Application What was Carly Patterson’s total for the events other than the floor exercise? Estimate by rounding to the nearest whole number. 9.7 + 9.3 + 9.45 The total is about 28 points. 10 + 9 + 9 = 28 First estimate the sum of 9.7, 9.3, and 9.45.

Additional Example 1A Continued Then add. 9.70 9.30 +9.45 28.45 Align the decimal points.Use zeros as placeholders.Add. Then place the decimal point. Since 28.45 is close to the estimate of 28, the answer is reasonable. Carly Patterson’s total for the events other than the floor exercise was 28.45.

Additional Example 1B: Sports Application How many more points did Carly need on the vault to have a perfect score of 10? 10.0 –9.3 0.7 Align the decimal points.Use zeros as placeholders.Subtract. Then place the decimal point. Carly needed another 0.7 of a point to have a perfect score. Find the difference between 10 and 9.3.

You can place any number of zeros at the end of a decimal number without changing its value. Helpful Hint

Check It Out: Example 1A What was Carly Patterson’s total for the events other than the vault exercise? Estimate by rounding to the nearest whole number. 9.7 + 9.7 + 9.45 The total is about 29 points. 10 + 10 + 9 = 29 First estimate the sum of 9.7, 9.7, and 9.45.

Check It Out: Example 1A Continued Then add. 9.700 +9.450 28.850 Align the decimal points.Use zeros as placeholders.Add. Then place the decimal point. Since 28.850 is close to the estimate of 29, the answer is reasonable. Carly Patterson’s total for the events other than the vault exercise was 28.85.

Check It Out: Example 1B How many more points did Carly need on the uneven bars to have a perfect score of 10? 10.00 –9.45 0.55 Align the decimal points.Use zeros as placeholders.Subtract. Then place the decimal point. Carly needed another 0.55 of a point to have a perfect score. Find the difference between 10 and 9.45.

Additional Example 2: Using Mental Math to Add and Subtract Decimals Find each sum or difference. A. 1.8 + 0.2 B. 4 – 0.7 Think: 0.8 + 0.2 = 1. 1.8 + 0.2 = 2.0 4 – 0.7 = 3.3 Think: 0.7 + 0.3 = 1, so 1 – 0.7 = 0.3.

Check It Out: Example 2 Find each sum or difference. A. 1.6 + 0.4 B. 6 – 0.3 Think: 0.6 + 0.4 = 1. 1.6 + 0.4 = 2.0 6 – 0.3 = 5.7 Think: 0.3 + 0.7 = 1, so 1 – 0.3 = 0.7.

Additional Example 3A: Evaluating Decimal Expressions Evaluate 6.73 – x for each value of x. x = 3.8 Substitute 3.8 for x. 6.73 – x 6.73 – 3.8 6.73 Align the decimal points. – 3.80 Use a zero as a placeholder. 2.93 Subtract. Place the decimal point.

Additional Example 3B: Evaluating Decimal Expressions Evaluate 6.73 – x for each value of x. x = 2.9765 Substitute 2.9765 for x. 6.73 – x 6.73 – 2.9765 6.7300 Align the decimal points. –2.9765 Use zeros as placeholders. 3.7535 Subtract. Place the decimal point.

Check It Out: Example 3A Evaluate 7.58 – x for each value of x. x = 3.8 Substitute 3.8 for x. 7.58 – x 7.58 – 3.8 7.58 Align the decimal points. –3.80 Use a zero as a placeholder. 3.78 Subtract. Place the decimal point.

Check It Out: Example 3B Evaluate 8.17 – x for each value of x. x = 2.9765 Substitute 2.9765 for x. 8.17 – x 8.17 – 2.9765 8.1700 Align the decimal points. – 2.9765 Use zeros as placeholders. 5.1935 Subtract. Place the decimal point.

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Standard Lesson Quiz Complete in your notebook ~ Lesson Quizzes

Lesson Quiz Find each sum or difference. 1. 8.3 + 2.7 2. 9.7 – 4 3. 22.6 + 8.4 4. Evaluate 12.76 – x for x = 8.41. 5. During an ice-skating competition, Dawn received the following scores: 4.8, 5.2, 5.4. What was Dawn’s total score? 5.7 11 31 4.35 15.4

HOMEWORK Class Periods 1, 4 & 7 Page 104, #14-29 Class Periods 2, 3 & 6 (Advanced) Page 100, 104, #14-29 & 36-39 on page 105

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