Adding and Subtracting DecimalsLesson 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)
28.913.31+
0
42.21The 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.
Example: 15 – 9.372
159.372-
.0004 9 9 10
826.
0 1
5
Homework Time
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