Adding and Subtracting Decimals

1
Adding and Subtracting Decimals

• Lesson 1-5

2
The Basic 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 5335
Associative Property of Addition Numbers can be
grouped in any way. Example (73) 5 7
(35)
Identity Property of Addition The sum of any
number and zero is that number. Example 18 0
18
4
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
5
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.

6
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.
7
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.

8
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 9s.
• The final zero becomes 10.

9
Example 15 9.372
4 9 9 10
0
1
.000
15
-
9.372
5
8
2
6
.
10
Homework Time
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