1. Lesson 4.2 Adding and Subtracting Decimals
November 7, 2007

2. Essential Question: How do I add and subtract decimals?
Always line up decimals
Add and subtract like you always do
Bring decimal straight down in your answers

3. You can add a zero to help keep everything lined up
Adding
Examples:
Put the first number on the top of the second number and line up the decimals.
You can add a zero to help keep everything lined up
When adding or subtracting you always start from the right and work left.
15.85

4. Add a zero to line up everything
When there is
not a decimal
put one
behind the
number.
Line up the decimal
Add a zero to line up everything
22.44

5. Add a zero to line up everything
-8.33
Line up
the decimal
Add a zero to line up everything +
Same signs
Add – keep the sign

6. Subtracting Examples: 5.34 -2.08 5.34 -2.08 3.26
Put the first number on the top of the second number and line up the decimals.
Different signs
Take difference
3.26

7. -38.59 – (-7.81) -38.59 + 7.81 -30.78 + Line up the decimal
Line up
the decimal +
-30.78
Subtract to add when negatives are used.
Different signs take difference

8. Add zeros to line up everything, then subtract
28 –
When there is
not a decimal
put one
behind the
number.
Add zeros to line up everything, then subtract 7 9 9
000 . 1 2 8 9

9. The sum of each row, column, and diagonal must be the same.
Magic Square
0.5 7
4.625
8.75
The sum of each row, column, and diagonal must be the same.
6.375
0.5 7
5.25
4.625 4
2.25
8.75
2.875
Sums to