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Diana Lenartiene, Ed. S. moderator/instructor

Elluminate Meeting/Classroom 2

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6/10/2008; updated: 10/3/11

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Adding and Subtracting Decimals

Solve problems involving addition and subtraction of decimals.

To add decimals

Line up the decimal points and add the columns from right to left.

Place a decimal point in the answer directly below the other decimal points.

Example 1

0.64 + 0.39Line up the decimal points.

0.64 + 0.39 __________________________

Add the columns. 0.64

+ 0.39 __________________________

1.03

Example 2

5.74 + 3.3Line up the decimal points. 5.74 + 3.3 _____________________________

Add the columns. 5.74 + 3.3 _____________________________

9.04

Try this one on your own!

39.66

+22.7

Remember, line up the decimals, Place the decimal point in the answer under the ones in the problem,Add like whole numbers.

11

39.66+22.7 62.36

To subtract decimals

Line up the decimal points.Subtract the columns from right to left,

regrouping if necessary.Place a decimal point in the answer

directly below the other decimal points.

Example 10.86 – 0.49

Line up the decimal points0.86

- 0.49 __________________________

Regroup the tenths. Subtract the columns.

0.86 - 0.49 __________________________

167

0.37

NOTE BOOK

You can use graph paper to model decimal multiplication. Here is a model of the product of 0.7 and 0.3.

1

1

0.3

0.7

Because 21 of the 100 squares are shaded, 0.7 0.3

Words

Numbers

Multiplying Decimals

= 0.21.

Multiply decimals as you would whole numbers. Then place the decimal point in the product. The number of decimal places in the product is equal to the sum of the number of decimal places in the factors.

0.7 0.3 = 0.21

Multiplying Decimals

EXAMPLE 1 Multiplying Decimals

Multiplying Decimals

5.82

0.41

2 decimal places

2 decimal places

5 8 22 3 2 8

4 decimal places

–––+––––––

2.3 8 6 2–––––––

1234

Checking Reasonableness To check that the product in Example 1 is reasonable, round each factor to the place value of the leading digit, then multiply. The leading digit of a number is the first nonzero digit.

Multiplying Decimals

5.82 6

0.41 0.4

Round to the nearest whole number.

Round to the nearest tenth.

Because 6 0.4 = 2.4, the product of the previous example is reasonable.

6.45

18

2 decimal places

0 decimal places

5160645

––––––116.10

Because 6 20 = 120, the product is reasonable.

EXAMPLE 2 Multiplying Decimals

–––+––––––

After you place the decimal point, you can drop any zeros at the end of an answer.

2 decimal places

CHECK

Multiplying Decimals

ANSWER 6.45 18 = 116.1

Write a zero before the 7 as a placeholder so that the number has 5 decimal places.

6.45

18

2 decimal places

0 decimal places

5160645

––––––116.10

Because 6 20 = 120, the product is reasonable.

1.273

0.06

3 decimal places

2 decimal places

76380.0 5 decimal places

Because 1 0.06 = 0.06, the product is reasonable.

EXAMPLE 2 Multiplying Decimals

–––+–––––– ––––––– –––+

After you place the decimal point, you can drop any zeros at the end of an answer.

2 decimal places

CHECK CHECK

Multiplying Decimals

ANSWER 6.45 18 = 116.1 ANSWER 1.273 0.06 = 0.07638

Central Park Central Park is a rectangular park about 2.5 miles long and about 0.5 mile wide. What is the area of Central Park?

EXAMPLE 3 Multiplying Decimals to Find Area

SOLUTION

Multiplying Decimals

= l w2.5(0.5)

A = l w

Substitute 2.5 for l and 0.5 for w.

= 1.25

Write formula for area of a rectangle.

ANSWER The area of Central Park is about 1.25 square miles.

Multiply.

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Thank you for viewing this presentation.Diana Lenartiene, IRSC ABE Instructor

If you still have questions, please contact me at:

dlenarti@irsc.edu