Number 8. Negative Numbers Mr F’s Maths Notes Number 8. Negative Numbers

8. Negative Numbers WARNING The Number Line – + If you are not concentrating, negative numbers can trip up the best of mathematicians. So… have a glass of water, shake all other thoughts out of your head, sit down, take a deep breath, and let’s begin… The Number Line The key to negative numbers is the number line. Now, I like to think of the number line going up and down, so when you add you go up, and when you subtract, you go down. Kind of like a thermometer. If you ever find yourself stuck or unsure about a negative number question, just draw yourself a very quick number line, count the spaces with your finger, and you will be fine. I still do this, and I’m… well, quite a bit older then you. 7 4 3 2 1 -1 -2 -3 6 5 4 3 2 – + 1 - 1 - 2 - 3 - 4 - 5 - 6

Adding and Subtracting when the Signs are NOT Touching Where people seem to go wrong with negative numbers is that they learn the rule that two minuses make a plus. Now, this rule is a good one, but must only be used when two signs (+ or -) are touching. If no signs are touching, I would just use this rule Rule: If no signs are touching, use a number line (on paper or in your head), or think about money! Example 1 Example 2 Example 3 7 6 2 - 7 -4 + 6 -1 - 4 5 Number Line: put your finger at 2 and move down 7 places Number Line: put your finger at -4 and move up 6 places Number Line: put your finger at -1 and move down 4 places 4 3 2 – + Money: if I have £2 in my bank and someone takes away £7, then how much do I have? Money: if I have £-4 in my bank (I am in debt) and someone gives me £6, then how much do I have? Money: if I have £-1 in my bank (I am in debt) and someone takes away £4, then how much do I have? 1 - 1 - 2 - 3 2 – 7 = -5 -4 + 6 = 2 -1 - 4 = -5 - 4 - 5 - 6

Now both these methods still work when the numbers become harder and the number line becomes too big to draw: Example 4 Example 5 56 56 - 89 -102 + 217 Number Line: imagine your finger is as 56. How far must you go down to get to zero?... 56 spaces, right? And so, how much further down do you still have to go?… another 33 spaces! 115 Number Line: imagine your finger is as -102. How far must you go up to get to zero?... 102 spaces, right? And so, how much further up do you still have to go?… another 115 spaces! –56 +115 –33 +102 Money: if I have £56 in my bank and someone takes away £89, then how much do I have? Money: if I have £-102 in my bank and someone gives me £217, then how much do I have? - 33 - 102 56 – 89 = -33 -102 + 217 = 115

+ and - = - - and + = - + and + = + - and - = + Adding and Subtracting when the Signs ARE Touching Okay, now it’s time for our rule… Rule: If two signs are touching (+’s or –’s next to each other), then replace the two signs with one sign using these rules: + and - = - - and + = - + and + = + - and - = + Example 1 Example 2 Example 3 Example 4 -4 + -8 5 - -6 -22 - - 9 -6 - 10 Have you spotted the touching signs?... Using our rule, we can change + and - to - So, our sum becomes: Have you spotted the touching signs?... Using our rule, we can change - and - to + So, our sum becomes: Have you spotted the touching signs?... Using our rule, we can change - and - to + So, our sum becomes: Have you spotted the touching signs?... I hope not, because there aren’t any! The two minuses are NOT touching So our sum stays the same and we do it using either of our methods -4 - 8 5 + 6 -22 + 9 Which is pretty easy using either number lines or money. Which is pretty easy however you do it! Which is pretty easy using either number lines or money -4 + -8 = -12 5 - -6 = 11 -22 - -9 = -13 -6 - 10 = -16

Multiplying and Dividing As was the case with fractions, multiplying and dividing with negative numbers is a little bit easier than adding and subtracting, but you still have to concentrate! Rule: Do the sum as normal, ignoring the plus and minus signs and write down the answer Then, carefully count the number of minus signs in the question. If there is one the whole answer is negative, if there are two the answer is positive, if there are three the answer is negative, four means positive, and so on… Example 1 Example 2 Example 3 Example 4 -20 ÷ 4 -6 x -9 -3 x -2 x -5 Do the sum as normal, ignoring the minus signs Do the sum as normal, ignoring the minus signs Do the sum as normal, ignoring the minus signs Do the sum as normal, ignoring the minus signs 20 ÷ 4 = 5 6 x 9 = 54 3 x 2 x 5 = 30 Count the number of minus signs in the question… 1! Count the number of minus signs in the question… 2! Count the number of minus signs in the question… 3! Count the number of minus signs in the question… 2! One minus makes the whole answer negative So: Two minuses makes the whole answer positive So: Three minuses makes the whole answer negative So: Two minuses makes the whole answer positive So: -20 ÷ 4 = -5 -6 x -9 = 54 -3 x -2 x -5 = -30

Tricky Questions involving Negative Numbers The people who write maths exams are nasty. Just when you think you have got a topic sorted, they chuck in a right stinker. But do not panic. So long as you remember the rules we have discussed here, and you don’t forget our old friend BODMAS/BIDMAS, you will be fine! Example 1 Example 2 Example 3 Now, BIDMAS says we must sort out the brackets first: Now, BIDMAS says we must sort out the division first: Now remember, even though we can’t see any brackets, they are hidden on the top and bottom of the fraction: So now we have: Putting that back in the question, we have: So, the top gives us: And using our negative number rules, we should get the answer of: Let’s sort those two touching signs out: And from the bottom: = -4 Using number lines, or money, we should get Leaving us with: = -7 = 2

Good luck with your revision!