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# Adding and Subtracting Decimals - PowerPoint PPT Presentation

Adding and Subtracting Decimals. Lesson 1-5. The Basic Steps:. Line up the numbers by the decimal point. Fill in missing places with zeroes. Add or subtract. Addition Properties. Commutative Property of Addition: Numbers can be added in any order. Example: 5+3=3+5.

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Presentation Transcript

Lesson 1-5

• Line up the numbers by the decimal point.

• Fill in missing places with zeroes.

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

• Addition properties can be used to quickly find sums.

• Example: What is 52 + 12.4 + 48?

• 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

• Before adding your numbers, use one of the estimating strategies you learned to see what the answer should be close to.

This can estimate to 30 + 13, which is 43. (Compatible numbers)

28.9

0

+

13.31

42.21

The answer is close to the estimate.

• 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.

• 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.

0

1

4 9 9 10

.000

15

-

9.372

5

.

6

2

8

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