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Adding and Subtracting Decimals

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Presentation on theme: "Adding and Subtracting Decimals"— Presentation transcript:

1 Adding and Subtracting Decimals

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

3 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

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

5 Why Estimate? Before adding your numbers, use one of the estimating strategies you learned. When you add, you can see if your answer is reasonable.

6 Example: This can estimate to , which is 43. (Compatible numbers) 28.9 + 13.31 42.21 The answer is close to the estimate.

7 Subtracting The addition properties do not work for subtraction.
Estimate and subtract to find the answer to which it is closest. Subtract to find the real answer.

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

9 Example: 15 – 9.372 1 15 .000 - 9.372 5 . 6 2 8

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