Succeed with maths – Part 1

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1.1 Investigating multiplication by a negative number

It’s time to turn your observations from the previous activities into some useful mathematical rules to go with the ones that you already have for negative numbers from Week 7. This is the kind of process that you will often come across if you decide to undertake university-level study – and is an important part of problem solving in many different situations, not just in maths.

Activity 3 Being a mathematical detective

Timing: Allow approximately 5 minutes

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

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

If you multiply a negative number by a positive number, the answer is negative: (–) × (+) = (–).

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.

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

If you multiply a negative number by a negative number, the answer is positive: (–) × (–) = (+).

Both of the rules in this activity also work for division! Try dividing 20 by (–5), and (–20) by (–5) to confirm that this is the case.

Now you have these rules, it is helpful for your understanding to look a bit closer at what is actually happening.

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