Transcript Multiply Rational Numbers

Objective
The student will be able to:
multiply rational numbers.
Multiplying Rules
1) If the numbers have the same
signs then the product is positive.
(-7) • (-4) = 28
2) If the numbers have different
signs then the product is negative.
(-7) • 4 = -28
Examples
1) (3x)(-8y)
-24xy
2)
4 

20 
5
Write both numbers as a fraction.
Cross-cancel if possible.
 20 4 
 1 5 
Multiplying fractions:
80

5
top # • top #
= -16
Bottom # • bottom#
When multiplying two negative
numbers, the product is negative.
1. True
2. False
Answer Now
When multiplying a negative number
and a positive number, use the sign of
the larger number.
1. True
2. False
Answer Now
2
5




3) 

 5  8 
10
=
40
=
1
4
Multiply: (-3)(4)(-2)(-3)
1.
2.
3.
4.
72
-72
36
-36
Answer Now
an easy way to determine the sign of the answer
When you have an odd number of negatives,
the answer is negative.
When you have an even number of negatives,
the answer is positive.
4) (-2)(-8)(3)(-10)
Do you have an even or odd number of
negative signs?
3 negative signs -> Odd -> answer is negative
-480
Last one!
1
2



2
5)  65
 2 
 5 
Positive or negative answer?
Positive - even # of negative signs (4)
Write all numbers as fractions and multiply.
 1  6 52  2 
 2  1 1  5  1 
120
10
=12
What is the sign of the product of
(-3)(-4)(-5)(0)(-1)(-6)(-91)?
1.
2.
3.
4.
Positive
Negative
Zero
Huh?
Answer Now