Factorising Quadratics (Edexcel GCSE Maths: Foundation)

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Factorising Simple Quadratics

What is a quadratic expression?

  • A quadratic expression is an algebraic expression where the highest power of the unknown is 2
    • The general form for a quadratic expression is a x squared plus b x plus c
    • You will only come across quadratic expressions where a equals 1
    • Remember that b or c could have a value of 0 

How do I factorise a quadratic expression?

  • You can factorise a quadratic through inspection
  • If c  is positive then the factor pair must have either two positive factors or two negative factors 
    • A positive multiplied by a positive gives a positive
    • A negative multiplied by a negative gives a positive 
  • E.g. Factorise x squared plus 5 x plus 4
    • Identify all the factor pairs of c
      • In this example c = +4
      • Factor pairs for this example include:
        1, 4 or -1, -4
        2, 2 or -2, -2
    • Identify the factor pair that add together to give b
      • In this example, b = +5
      • Because of this we know that we are going to use a factor pair where both factors are positive
      • The factor pair that adds to 5 is 1 and 4 
    • Write a pair of brackets each with x  and one of the factors
      • (x  + 1)(x  + 4)

How do I factorise a quadratic where is positive but is negative? 

  • If c  is positive then the factor pair must have either two positive factors or two negative factors 
    • A positive multiplied by a positive gives a positive
    • A negative multiplied by a negative gives a positive
  • E.g. Factorise x squared minus 8 x plus 15
    • Identify all the factor pairs of c
      • In this example, c = +15
      • Factor pairs of +15 include:
        1, 15 or -1, -15
        3, 5 or -3, -5
    • Identify the factor pair that add together to give b
      • In this example, b = -8
      • Because of this we know that we are going to use a factor pair where the factors are both negative
      • The factor pair that adds to -8 is -3 and -5
    • Write a pair of brackets each with x  and one of the factors
      • (x  -3)(x  - 5)

How do I factorise a quadratic where is negative? 

  • If c  is negative then the factor pair must have one positive factor and one negative factor
    • A positive multiplied by a negative gives a negative
  • E.g. Factorise x squared minus 2 x minus 8
    • Identify all the factor pairs of c
      • In this example, c = -8
      • Factor pairs of -8 include:
        -1, +8 or +1, -8
        +2, -4 or -2, +4
    • Identify the factor pair that add together to give b
      • In this example, b = -2
      • The factor pair that adds to -2 is +2, -4
    • Write a pair of brackets each with x  and one of the factors
      • (x  + 2)(x  - 4)

Examiner Tip

  • As a check, expand your answer and make sure you get the same expression as the one you were trying to factorise.

Worked example

(a) Factorise x squared minus 4 x minus 21.

Factorise the expression by inspection

We need two numbers that multiply to -21 and sum to -4

Identify all factor pairs of -21

+1, - 21
-1, +21
+3, -7
-3, +7

Identify the factor pair that sum to -4

+3, -7

Write down the brackets 

 (x + 3)(x - 7)

 

Difference Of Two Squares

What is the difference of two squares?

  • When a 'squared' quantity is subtracted from another 'squared' quantity, you get the difference of two squares
    • for example,
      • a2 - b2
      • 92 - 52

 How do I factorise the difference of two squares?

  • The difference of two squares, a2 - b2, factorises to
    • open parentheses a plus b close parentheses open parentheses a minus b close parentheses
  • You can see why this is if you work backwards and expand the brackets (a + b)(a - b)
    •  (a  + b)( - b) = a2 - ab  + ba - b2
      • ab is the same quantity as ba, so -ab  and +ba  cancel out
    • (a + b)(a - b) = a2 - b2

Examiner Tip

  • The word difference in maths means a subtraction, it should remind you that you are subtracting one squared term from another.
  • The phrase 'difference of two squares' is not often used in exam questions.
    • Look out for questions that involve quadratics with only 2 terms and a subtraction sign between them!

Worked example

(a)
Factorise  x squared minus 16.

 

Recognise that x squared and 16 are both squared terms
Also, the second term is subtracted from the first term

Factorise using the difference of two squares

x squared minus 16 equals open parentheses x close parentheses squared minus open parentheses 4 close parentheses squared

Put the square root of each term added together in the first bracket
Put the square root of each term subtracted from each other in the second bracket

stretchy left parenthesis x plus 4 stretchy right parenthesis stretchy left parenthesis x minus 4 stretchy right parenthesis

 

(b)
Factorise 4 x squared minus 25.

Recognise that 4 x squared and 25 are both squared terms
Also, the second term is subtracted from the first term

Factorise using the difference of two squares

4 x squared minus 25 equals open parentheses 2 x close parentheses squared minus open parentheses 5 close parentheses squared

Put the square root of each term added together in the first bracket
Put the square root of each term subtracted from each other in the second bracket

stretchy left parenthesis 2 x plus 5 stretchy right parenthesis stretchy left parenthesis 2 x minus 5 stretchy right parenthesis

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Mark

Author: Mark

Expertise: Maths

Mark graduated twice from the University of Oxford: once in 2009 with a First in Mathematics, then again in 2013 with a PhD (DPhil) in Mathematics. He has had nine successful years as a secondary school teacher, specialising in A-Level Further Maths and running extension classes for Oxbridge Maths applicants. Alongside his teaching, he has written five internal textbooks, introduced new spiralling school curriculums and trained other Maths teachers through outreach programmes.