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1-2 Adding and Subtracting Decimals

1-2 Adding and Subtracting Decimals. Do Now: Vocabulary Review: What is place value? Write the value of the digit 8 in each number. 2. 0.385 3. 1.879 4. 812.4 5. 0.23. Homework Answers 1-1. $6.50 $3.00 $19.50 $4.50 11 5 9 6 12 10. 13 11. 3 12. 10 13. 5

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1-2 Adding and Subtracting Decimals

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  1. 1-2 Adding and Subtracting Decimals Do Now: Vocabulary Review: What is place value? Write the value of the digit 8 in each number. 2. 0.385 3. 1.879 4. 812.4 5. 0.23

  2. Homework Answers 1-1 • $6.50 • $3.00 • $19.50 • $4.50 • 11 • 5 • 9 • 6 • 12 10. 13 11. 3 12. 10 13. 5 14-19 Sample answers: 14. $346; rounding 15. 20; rounding 16. 4,500; rounding 17. 1,100; rounding 18. 12; compatible #s 19. 42; front-end estimation 20. 13; compatible #s 21. About $480 22. About 33,600 slices

  3. Guided Problem Solving 1-1 • 1. eight postcards; $.59 each; $2 total • 2. Estimate the total cost of eight postcards and stamps. • 3. The product of 8 and $.59 • 4. $.60 • 5. $4.80 • 6. $2 • 7. $4.80 1 $2 5 $6.80 • 8. yes, 8 3 0.59 1 2 5 6.72, which is close to 6.80 • 9. about $4.80

  4. Addition Properties Commutative Property of Addition: Numbers can be added in any order. Example: 5+3=3+5 Associative Property of Addition: Numbers can be grouped in any way. Example: (7+3) + 5 = 7 + (3+5) Identity Property of Addition: The sum of any number and zero is that number. Example: 18 + 0 = 18

  5. The Basic Steps: • Line up the numbers by the decimal point. • Fill in missing places with zeroes. • Add or subtract.

  6. Mental Math • Addition properties can be used to quickly find sums. • Example: What is 52 + 12.4 + 48? • We can use an addition property to quickly add this. • First, change the order: 52 + 48 + 12.4 ________________ property. • Then, group the first two addends together: (52 + 48) + 12.4________________ property • 100 + 12.4 = 112.4 commutative associative

  7. Why Estimate? • Before adding your numbers, use one of the estimating strategies you learned to see what the answer should be close to. • When you add, you can see if your answer is reasonable.

  8. Example: 28.9 + 13.31 This can estimate to 29 + 13, which is 42. (Rounding) 28.9 0 + 13.31 42.21 The answer is close to the estimate.

  9. Subtracting • The addition properties do not work for subtraction. • Estimate and subtract to find what the answer is close to. • Subtract to find the real answer.

  10. Subtracting Across Zeroes • If you have several zeroes in a row, and you need to borrow, go to the first digit that is not zero, and borrow. • All middle zeroes become 9’s. • The final zero becomes 10.

  11. Example: 15 – 9.372 0 1 4 9 9 10 .000 15 - 9.372 5 . 6 2 8

  12. A marching band has 6.5 minutes to perform at a football game. Does the band have enough time to play “America the Beautiful” and Yankee Doodle Dandy”? • How long will the band take to play “Stars and Stripes Forever” and “The Star Spangled Banner”?

  13. A marching band has 6.5 minutes to perform at a football game. Does the band have enough time to play “America the Beautiful” and Yankee Doodle Dandy”? • The band will play for 6.13 minutes. The band will have enough time. • 2. How long will the band take to play “Stars and Stripes Forever” and “The Star Spangled Banner”? • 8.02 min

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