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A LevelMaths: PureAQARevision Notes8. Integration8.2 Further Integration8.2.5 Substitution (Reverse Chain Rule)

Substitution (Reverse Chain Rule) (AQA A Level Maths: Pure)

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Substitution (Reverse Chain Rule)

What is integration by substitution?

  • Make sure you are familiar with Chain Rule and Reverse Chain Rule
  • In more awkward problems it is harder to spot the reverse chain rule
  • It is possible to use a substitution
  • These kinds of substitutions usually will not be given
    • The substitution is deemed ‘obvious’Notes subst_eq, AS & A Level Maths revision notes 

How do I integrate when a substitution is not given?

  • Look to substitute the ‘second’ (rather than the ‘main’) function
  • STEP 1: Determine the substitution
    • What is the ‘main’ function? ‘second’ function?

  • STEP 2: Differentiate the substitution and rearrange
    • du/dx here can be treated like a fraction (eg × dx to get rid of fractions)

  • STEP 3: Replace all parts of the integral
    • all x terms should be replaced with equivalent u terms, including dx
    • if a definite integral change the limits from x to u too

  • STEP 4: Integrate and either …
    • …substitute x back in

      or

    • … evaluate the definite integral using the u limits

  • STEP 5: Find c, the constant of integration, if needed

  Notes subst_eg_soln2, AS & A Level Maths revision notes

Notes subst_eg2_def1, AS & A Level Maths revision notes Notes subst_eg2_def2, AS & A Level Maths revision notes

Examiner Tip

  • A lot of the “work” in these problems happens in your head :
    • what is the ‘main’ function?
    • what should the substitution be?
    • are there any numbers to ‘adjust’ and ‘compensate’ for?
  • Be sure that what you write down is clear and easy to follow, and remember that you can check your final answer by differentiating it.

Worked example

Example soltn1, AS & A Level Maths revision notesExample soltn2, AS & A Level 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.