Composite Functions (CIE A Level Maths: Pure 1)

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Composite Functions

What is a composite function?

  • A composite function is where one function is applied after another function

Language of Functions Notes Diagram 8, A Level & AS Level Pure Maths Revision Notes

  • The ‘output’ of one function will be the ‘input’ of the next one
  • Sometimes called function-of-a-function
  • A composite function can be denoted
    • space f g left parenthesis x right parenthesis
    • space f stretchy left parenthesis g left parenthesis x stretchy right parenthesis right parenthesis
    • f open square brackets g left parenthesis x right parenthesis close square brackets
    • left parenthesis f ring operator g right parenthesis left parenthesis x right parenthesis
    • All of these mean “f of g left parenthesis x right parenthesis” 

Composite Functions Notes Diagram 1, A Level & AS Level Pure Maths Revision Notes 

How do I work with composite functions?

Composite Functions Notes Diagram 2, A Level & AS Level Pure Maths Revision Notes

  • Recognise the notation
    • fg(x) means “f of g of x”
    The order matters
    • First apply g to x to get space g left parenthesis x right parenthesis
    • Then apply f to the previous output to get space f stretchy left parenthesis g left parenthesis x stretchy right parenthesis right parenthesis
    • Always start with the function closest to the variable
    • f g left parenthesis x right parenthesis is not usually equal to g f left parenthesis x right parenthesis

Special cases

Composite Functions Notes Diagram 3, A Level & AS Level Pure Maths Revision Notes

  • fg(x) and gf(x) are generally different but can sometimes be the same
  • ff(x) is written as f2(x)
  • Inverse functions ff-1(x) = f-1f(x) = x

 

Examiner Tip

  • Domain and range are important.In fg(x), the ‘output’ (range) of g must be in the domain of f(x), so fg(x) could exist, but gf(x) may not (or not for some values of x). 

Worked example

Composite Functions Example Diagram, A Level & AS Level Pure Maths Revision Notes

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Paul

Author: Paul

Expertise: Maths

Paul has taught mathematics for 20 years and has been an examiner for Edexcel for over a decade. GCSE, A level, pure, mechanics, statistics, discrete – if it’s in a Maths exam, Paul will know about it. Paul is a passionate fan of clear and colourful notes with fascinating diagrams – one of the many reasons he is excited to be a member of the SME team.