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Negative Numbers Simplified Revision Notes

Revision notes with simplified explanations to understand Negative Numbers quickly and effectively.

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Negative Numbers

Understanding the Number Line:

The key to understanding negative numbers is the number line. Imagine a number line where positive numbers are to the right of zero, and negative numbers are to the left.

  • Positive Numbers: Increase as you move to the right.
  • Negative Numbers: Decrease as you move to the left.

Think of the number line like a thermometer:

  • Going up the number line (to the right) increases the value.
  • Going down the number line (to the left) decreases the value.
image

Adding and Subtracting when Signs are NOT Touching

When you add or subtract numbers with different signs and they are not directly next to each other (i.e., no two negative signs touching), the key is to visualize the process using a number line or to think about it in terms of money.

Number Line Approach

Imagine a number line where positive numbers increase as you move to the right and negative numbers decrease as you move to the left.

Worked Examples:

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Example 1: 272−7

  1. Start at 22 on the number line.
  2. Move left 77 spaces because you are subtracting 77.
  3. Final position is at 5-5.
27=52−7=−5

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Example 2: 4+6−4+6 4. Start at 4-4 on the number line. 5. Move right 66 spaces because you are adding 66. 6. Final position is at 22.

4+6=2−4+6=2

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Example 3: 14−1−4 7. Start at 1-1 on the number line. 8. Move left 44 spaces because you are subtracting 44. 9. Final position is at 5-5.

14=5−1−4=−5

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Example 4: 568956−89 10. Visualize Starting at 5656:

  • Imagine your finger is at 5656 on the number line.
  1. Move Left to Zero:
  • To reach zero, you must move 5656 spaces to the left.
  1. Move Further Left:
  • You still need to subtract more, so continue 3333 more spaces left to complete the subtraction (since 8956=3389−56=33).
  1. Final Position:
  • Your final position on the number line is at 33−33.
5689=3356−89=−33

Money Analogy:

  • Imagine you have £56£56 and someone takes away £89£89. You lose all £56£56, and you're now £33£33 in debt.

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Example 5: 102+217−102+217 14. Visualize Starting at 102−102:

  • Imagine your finger is at 102−102 on the number line.
  1. Move Right to Zero:
  • To reach zero, you must move 102102 spaces to the right.
  1. Move Further Right:
  • You still need to add more, so continue 115115 more spaces right to complete the addition (since 217102=115217−102=115).
  1. Final Position:
  • Your final position on the number line is at 115115.
102+217=115−102+217=115

Money Analogy:

  • Imagine you are £102£102 in debt, and someone gives you £217£217. You first pay off your debt and are left with £115£115.

Adding and Subtracting When Signs ARE Touching

The Rule for Touching Signs

Rule: If two signs are touching (meaning a ++ or - is directly next to another ++ or -), you can replace them with a single sign using the following rules:

  • and+=- and + = -
  • +and=+ and - = -
  • and=+- and - = +
  • +and+=++ and + = +

This rule simplifies calculations and helps avoid common mistakes when working with negative numbers.

Worked Examples:

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Example 1: 4+(8)−4+(−8) Identify the Touching Signs:

  • Here, ++ and - are touching. Apply the Rule:

       •  Replace :highlight[$+ -$ with $-$].
    

Simplify the Expression:

  • The sum now becomes 48−4−8. Calculate the Result:

  • Using a number line or mentally, move 88 spaces left from 4-4.

48=12−4−8=−12

Final Answer: 4+(8)=12−4+(−8)=−12

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Example 2: 5(6)5−(−6) Identify the Touching Signs:

  • Here, - and - are touching. Apply the Rule:

  • Replace - - with ++. Simplify the Expression:

  • The sum now becomes 5+65+6. Calculate the Result:

  • Add 66 to 55.

5+6=115+6=11

Final Answer: 5(6)=115−(−6)=11

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Example 3: 22(9)−22−(−9) Identify the Touching Signs:

  • Here, - and - are touching. Apply the Rule:

  • Replace - - with ++. Simplify the Expression:

  • The sum now becomes 22+9−22+9. Calculate the Result:

  • Add 99 to 22-22 by moving 99 spaces right from 22-22.

22+9=13−22+9=−13

Final Answer: 22(9)=13−22−(−9)=−13

Check the Signs:

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Example 4: 610−6−10

  • Here, the signs are not touching, so we don't apply the rule. Proceed with Calculation:

  • The sum remains 610−6−10. Calculate the Result:

  • Move 1010 spaces left from 6-6.

610=16−6−10=−16

Final Answer: 610=16−6−10=−16


Multiplying and Dividing

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Key Rule:

  1. Perform the multiplication or division as you normally would, ignoring the plus and minus signs.
  2. Count the number of minus signs in the problem:
  • If there is one minus sign, the result is negative.
  • If there are two minus signs, the result is positive.
  • If there are three minus signs, the result is negative.
  • If there are four minus signs, the result is positive.
  • And so on...

Worked Examples:

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Example 1: 20÷4−20÷4 18. Perform the Division:

20÷4=520÷4=5
  1. Count the Minus Signs:
  • There is one minus sign in the problem.
  1. Determine the Sign:
  • One minus makes the answer negative. Final Answer: 20÷4=5−20÷4=−5

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Example 2: 6×9−6×−9 21. Perform the Multiplication:

6×9=546×9=54
  1. Count the Minus Signs:
  • There are two minus signs in the problem.
  1. Determine the Sign:
  • Two minuses make the answer positive. Final Answer: 6×9=54−6×−9=54

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Example 3: 3×2×5−3×−2×−5 24. Perform the Multiplication:

3×2×5=303×2×5=30
  1. Count the Minus Signs:
  • There are three minus signs in the problem.
  1. Determine the Sign:
  • Three minuses make the answer negative. Final Answer: 3×2×5=30−3×−2×−5=−30

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Example 4: 884\frac{−88}{−4} 27. Perform the Division:

88÷4=2288÷4=22
  1. Count the Minus Signs:
  • There are two minus signs in the problem.
  1. Determine the Sign:
  • Two minuses make the answer positive. Final Answer: 884=22\frac{−88}{−4}=22

Tricky Questions Involving Negative Numbers

Worked Examples:

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📑Example 1: 2×(3+5)-2 \times (3 + 5) 3. Apply BIDMAS/BODMAS:

  • Brackets first: Solve the expression inside the brackets.
3+5=83 + 5 = 8
  • Now, the expression becomes 2×8.−2×8.
  1. Multiply:
  • Perform the multiplication:
2×8=16-2 \times 8 = -16

Final Answer: 2×(3+5)=16-2 \times (3 + 5) = -16


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📑Example 2: 3+8÷42-3 + \frac{-8 \div 4}{-2} 5. Apply BIDMAS/BODMAS:

  • Division first:
8÷4=2-8 \div 4 = -2
  • Substituting back into the expression gives:
3+22-3 + \frac{-2}{-2}
  1. Simplify the Fraction:
  • The fraction 22=1\frac{-2}{-2} = 1 (because a negative divided by a negative is positive).
  1. Combine with the Original Expression:
3+1=2-3 + 1 = -2

Final Answer: 3+8÷42=2-3 + \frac{-8 \div 4}{-2} = -2


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📑Example 3: 4×33(9)\frac{-4 \times -3}{-3 - (-9)} 8. Apply BIDMAS/BODMAS:

  • Multiplication first on the numerator:
4×3=12-4 \times -3 = 12
  • Simplify the denominator by handling the negative signs:
3(9)=3+9=6-3 - (-9) = -3 + 9 = 6
  • Now, the expression becomes:
126\frac{12}{6}
  1. Simplify the Fraction:
126=2\frac{12}{6} = 2

Final Answer: 4×33(9)=2\frac{-4 \times -3}{-3 - (-9)} = 2


Worked Example: Exam-Style Question

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📝Question: Simplify the expression 5×(24)+3−5×(2−4)+3

Step-by-Step Solution:

  1. Solve the Brackets:
24=22−4=−2
  • Now the expression becomes:
5×2+3−5×−2+3
  1. Multiply:
5×2=10−5×−2=10
  1. Add:
10+3=1310+3=13

Final Answer: 5×(24)+3=13−5×(2−4)+3=13

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