Lesson 4.2 Adding and Subtracting Decimals

November 7, 2007

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

Adding

Examples: 4.55 + 11.3

Put the first number on the top of the second number and line up the decimals.

4.55

+ 11.3

0 15.85

You can add a zero to help keep everything lined up When adding or subtracting you always start from the right and work left.

Line up the decimal 6.44 + 16 When there is not a decimal put one behind the number.

6.44

+ 16.

0 22.44

Add a zero to line up everythin g

-8.33 + -10.972

Line up the decimal + -8.33

0 -10.972

-19.302

Add a zero to line up everythin g Same signs Add – keep the sign

Examples:

Subtracting

5.34 -2.08

Put the first number on the top of the second number and line up the decimals.

5.34

-2.08

3.26

Different signs Take difference

-38.59 – (-7.81)

Line up the decimal + -38.59

+ 7.81

-30.78

Subtract to add when negatives are used.

Different signs take difference

28 – 15.911

When there is not a decimal put one behind the number.

7 9 9 28.

000 -15.911

1 2 .

0 8 9 Add zeros to line up everything, then subtract

The sum of each row, column, and diagonal must be the same.

Magic Square 0.5

4.625

8.75

7 6.375

0.5

5.25

4.625

2.25

8.75

Sums to 13.875

7 4 2.875