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.

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Adding and Subtracting Decimals

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Adding and subtracting decimals

Adding and Subtracting Decimals

Lesson 1-5


The basic steps

The Basic Steps:

  • Line up the numbers by the decimal point.

  • Fill in missing places with zeroes.

  • Add or subtract.


Addition properties

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

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

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

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

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

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.


Example 15 9 372

Example: 15 – 9.372

0

1

4 9 9 10

.000

15

-

9.372

5

.

6

2

8


Classwork

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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