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Objective: By the end of class all of you will be able to add and subtract decimals by lining up the decimal point. • Essential Question(s): • What is the purpose of the decimal point? • How are decimals added or subtracted? 1 Adding and Subtracting Decimals Properties of Addition: Commutative Property ­ Changing the order of the addends does not change the sum. (Order Property) = 2 + 3 + 1 Ex. 1 + 2 + 3 3 + 3 5 + 1 6 6 Associative Property ­ (Grouping Property) Changing the grouping of the addends does not change the sum. Ex. (4 + 6) + 3 = 4 + (6 + 3) 10 + 3 4 + 9 13 13 Additive Identity Property ­ The sum of 0 and any number (Zero Property) is that number. Ex. 0 + 5 = 5 6 + 0 = 6 2 Adding and Subtracting Decimals: 1) Line up numbers vertically by their decimal points. 2) Equalize place values by adding zeros if needed. 3) Make an estimate of the sum 4) Add or Subtract each column regrouping if needed. 5) Compare your estimate to the actual to determine if reasonable. Ex. 8.5 + 0.65 + 1.5 Ex. 2.34 + 6.42 + 25 Ex. 16 ­ 8.79 Ex. 9.2 ­ 2.45 3