Idiot\'s Guides Basic Math and Pre-Algebra

Chapter 3: Order of Operations and Integers 43

To multiply signed numbers, multiply the absolute vales and follow these rules for signs.

Positive v Positive = Positive

Positive v Negative = Negative v Positive = Negative

Negative v Negative = Positive

If you’re multiplying more than two numbers, you can save some time by counting the number
of negative signs. If the number of negatives is even, the product will be positive. If the
number of negatives is odd, the product will be negative. For example, -2 v -5 v -3 v -1 =
(-2 v -5) v (-3 v -1) = 10 v 3 = 30. Four negatives, an even number, make a positive. But put
one more negative into the problem and you have -2 v -5 v -3 v -1 v -4 =
(-2 v -5) v (-3 v -1) v -4 = 10 v 3 v -4= 30 v -4 = -120. With an odd number of negatives,
your answer is negative.

WORLDLY WISDOM

A quick way to remember the rules for multiplying integers: multiply same signs, your

answer is positive; multiply different signs, it’s negative.

CHECK POINT

Complete each multiplication problem.

26. -4 v 30


27. 8 v -12


28. -7 v 15
    29. -11 v -43
    30. -250 v 401

You’ll be pleased to know that the rules for division of signed numbers are the same as the rules
for multiplication, except that, obviously, you divide the absolute values instead of multiplying.

To divide signed numbers, divide the absolute values and follow these rules for signs.

Positive z Positive = Positive

Positive z Negative = Negative z Positive = Negative

Negative z Negative = Positive

Following those rules, you can see that 42 z 6 = 7, and 84 z -4 = -21. -15 z 5 = -3,
but -15 z -5 = 3.