multiplying decimals i can multiply decimals to multiply: you do not line up the factors by the...
TRANSCRIPT
Multiplying Decimals
I can multiply decimals
To Multiply: You do not line up the factors by the decimal. Instead, place the number with more digits on
top. Line up the other number underneath, at the
right. Multiply Count the number of decimal places (from the
right) in each factor. Use the total number of decimal places in your
two factors to place the decimal in your product.
Multiply Decimals
Step 1 --- Ignore the decimal Step 2 --- Rewrite the problem Step 3 --- Multiply the problem Step 4 --- Underline all numbers to the right
of decimal(s) in original problem Step 5 --- Move decimal that many places left
in your answer
Steps 1 & 2
Ignore decimals and rewrite the problem
4 . 5 6
X 0 . 3 9
4 5 6
x 3 9
Step 3
Multiply problem without decimals
4 5 6
x 3 9
4 1 0 4
+ 1 3 6 8 0
1 7 7 8 4
Step 4
Underline numbers to the right of decimals in original problem
4. 5 6
x 0. 3 9
--------------- ( 4 places)
Step 5 Move decimal that many places left in answer
o 4 5 6o x 3 9o 4 1 0 4o + 1 3 6 8 0o 1 7 7 8 4 ( 4 places)
Multiplying Decimals
I can multiply decimals
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 80
4 decimal places
–––+––––––
2.3 8 6 2–––––––
1234
Example: 5.63 x 3.7
5.633.7x
1
2
4
4
39098
1
16+13
1
8
1
0
1
2
two
one
three.
6.45
18
2 decimal places
0 decimal places
51606450
––––––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.
Showdown
.31 x .7 = .217
.9 x .03 = .027
2 x .006 = .012
.66 x 2.52 = 1.6632