Adding and Subtracting Decimals
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
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
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: This can estimate to , which is 43. (Compatible numbers) 28.9 + 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.
Example: 15 – 9.372 1 15 .000 - 9.372 5 . 6 2 8
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