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Adding and Subtracting DecimalsPowerPoint Presentation

Adding and Subtracting Decimals

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### 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

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

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

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.

Example: 28.9 + 13.31

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.

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.

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.

Classwork

Use this site to play MATHFROG- ADDING AND SUBTRACTING FRACTIONS

http://cemc2.math.uwaterloo.ca/mathfrog/english/kidz/addsubdec.shtml

Homework Time!

P. 27, 1-49 odd

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