1. Adding and Subtracting Decimals Section 1-2 (pages 11-16)

2. Review Use to compare the numbers: 1. 203 230 2. 1,047 147 3. 989 998 4. 1, 260 126 < < > >

3. Identity Property of Addition The sum of zero and a is a. ArithmeticAlgebra 3.4 + 0 = 3.4a + 0 = a 0 + 3.4 = 3.40 + a = a

4. Commutative Property of Addition Changing the order of the addends does not change the sum. Arithmetic 4.1 + 1.7 = 1.7 + 4.1 Algebra a + b = b + a

5. Associative Property of Addition Changing the grouping of the addends does not change the sum. Arithmetic (2.6 + 4.3) + 8 = 2.6 + (4.3 + 8) Algebra (a + b) + c = a + (b + c)

6. Can we use these properties when we subtract? NO!!!!!!!! Subtraction is NOT commutative: 3.1 – 1.5 = 1.6 1.5 – 3.1 = -1.6 Subtraction is NOT associative: (4.0 – 2.4) – 6.3 = -4.7 4.0 – (2.4 – 6.3) = 7.8 Subtraction does NOT have an identity function: 7.8 – 0 = 7.8 0 – 7.8 = -7.8 NOT EQUAL!

7. Things to remember.. ALWAYS align your decimal points before you start. If needed, insert zeros so each addend has the same number of decimal places. Example: Find 5.6 + 1.23 + 4.578. 5.600 1.230 +4.578 11.408

8. Example 1 You guess the weight of a package is 10 lb. At the post office, the scale shows that the actual weight is 7.6 lb. Find the difference between your guess and the actual weight. ANSWER: 2.4 lb

9. Example 2 You place three songs on a CD with lengths of 3.2 min, 2.78 min, and 3.91 min. How time is taken up so far on the CD? ANSWER: 9.89 min

10. Additional Problems 1) 4.56 + 2.92) 5.3 – 0.123) 12.46 – 7.2 4) 7 - 2.45) 6.12 + 86) 0.75 + 3.8 + 4 7.465.185.26 4.612.128.55