Definite Integration (CIE A Level Maths: Pure 1)

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Definite Integration

What is definite integration?

  • Definite Integration occurs in an alternative version of the Fundamental Theorem of Calculus
  • This version of the Theorem is the one referred to by most AS/A level textbooks/websites

-Notes-fig1, AS & A Level Maths revision notes

  • a and b are called limits
    • a is the lower limit
    • b is the upper limit
  • f’(x) is the derivative of f(x)

What happened to c, the constant of integration?

Notes fig2, AS & A Level Maths revision notes

  •  “+c” would appear in both f(a) and f(b)
    • Since we then calculate f(b)f(a) they cancel each other out
    • There would be a “+c” from f(b) and a +c” from f(a)
  • So “+c” is not included with definite integration

How do I find a definite integral?

  • STEP 1: If not given a name, call the integral
    • This saves you having to rewrite the whole integral every time!
  • STEP 2:  If necessary rewrite the integral into a more easily integrable form
    • Not all functions can be integrated directly
  • STEP 3:  Integrate without applying the limits
    • Notation: use square brackets [ ] with limits placed after the end bracket
  • STEP 4:  Substitute the limits into the function and calculate the answer

Notes fig3, A Level & AS Level Pure Maths Revision Notes

Using a calculator

  • Advanced scientific calculators can work out the values of definite integrals
  • The button will look similar to:

Notes fig4, AS & A Level Maths revision notes

Notes fig5, AS & A Level Maths revision notes

  •  (Note how the calculator did not return the exact value open parentheses 1256 over 3 close parentheses of the integral)

Examiner Tip

  • Look out for questions that ask you to find an indefinite integral in one part (so “+c” needed), then in a later part use the same integral as a definite integral (where “+c” is not needed).

Worked example

Find the value of

integral subscript 2 superscript 4 3 x left parenthesis x squared minus 2 right parenthesis space straight d x


Start by expanding the brackets inside the integral

integral subscript 2 superscript 4 open parentheses 3 x cubed minus 6 x close parentheses space straight d x


Integrate as usual (here it's a 'powers of x' integration)

Write the answer in square brackets with the integration limits outside

table row cell integral subscript 2 superscript 4 open parentheses 3 x cubed minus 6 x close parentheses space straight d x end cell equals cell open square brackets 3 open parentheses fraction numerator x to the power of 3 plus 1 end exponent over denominator 3 plus 1 end fraction close parentheses minus 6 open parentheses fraction numerator x to the power of 1 plus 1 end exponent over denominator 1 plus 1 end fraction close parentheses close square brackets subscript 2 superscript 4 end cell row blank equals cell open square brackets 3 over 4 x to the power of 4 minus 3 x squared close square brackets subscript 2 superscript 4 end cell end table

Now substitute 4 into that function
And subtract from it the function with 2 substituted in

table attributes columnalign right center left columnspacing 0px end attributes row cell open square brackets 3 over 4 x to the power of 4 minus 3 x squared close square brackets subscript 2 superscript 4 end cell equals cell open parentheses 3 over 4 open parentheses 4 close parentheses to the power of 4 minus 3 open parentheses 4 close parentheses squared close parentheses minus open parentheses 3 over 4 open parentheses 2 close parentheses to the power of 4 minus 3 open parentheses 2 squared close parentheses close parentheses end cell row blank equals cell open parentheses 192 minus 48 close parentheses minus open parentheses 12 minus 12 close parentheses end cell row blank equals cell 144 minus 0 end cell row blank equals 144 end table

 

bold integral subscript bold 2 superscript bold 4 bold 3 bold italic x stretchy left parenthesis x squared minus 2 stretchy right parenthesis bold space bold d bold italic x bold equals bold 144

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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.