Adding and Subtracting Decimals Section 1 2 pages

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

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

Identity Property of Addition The sum of zero and a is a. Arithmetic 3. 4 + 0 = 3. 4 0 + 3. 4 = 3. 4 Algebra a+0=a 0+a=a

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

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)

Can we use these properties when we subtract? NO!!!! Subtraction does NOT have an identity function: Subtraction is NOT commutative: 3. 1 – 1. 5 = 1. 6 7. 8 – 0 = 7. 8 0 – 7. 8 = -7. 8 NOT EQUAL! 1. 5 – 3. 1 = -1. 6 NOT EQUAL! Subtraction is NOT associative: (4. 0 – 2. 4) – 6. 3 = -4. 7 4. 0 – (2. 4 – 6. 3) = 7. 8 NOT EQUAL!

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

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

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

Additional Problems 1) 4. 56 + 2. 9 7. 46 2) 5. 3 – 0. 12 5. 18 3) 12. 46 – 7. 2 5. 26 4) 7 - 2. 4 4. 6 5) 6. 12 + 8 12. 12 6) 0. 75 + 3. 8 + 4 8. 55