5.3.1 Investigating Multiplication by a Negative Number

Calculator symbol The calculator can be accessed on the left-hand side bar under Toolkit.

Now it’s time to turn detective, and write some mathematical rules.

Activity symbol Activity: Being a Mathematical Detective

Use the results from the tables for the previous activity to complete the following statements:

If you multiply a negative number by a positive number, the answer is …

Hint symbol

Discussion

Use the evidence from your tables that you wrote in your math notebook!

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Answer

If you multiply a negative number by a positive number, the answer is negative.

open minus close multiplication open plus closegives open minus close.

You can extend these rules a little. The order of the numbers doesn’t matter, so multiplying a positive number by a negative number also gives a negative answer. And the rules work for division too.

If you multiply a negative number by a negative number, the answer is …

Solution symbol

Answer

If you multiply a negative number by a negative number, the answer is positive. Again this rule also works for division.

open minus close multiplication open minus close gives open plus close.

Well done, you have discovered two other important mathematical rules about negative numbers!

5.3 Multiplication and Division with Negative Numbers

5.3.2 Rules for Multiplication and Division of Signed Numbers