Definite Integrals (Cambridge O Level Additional Maths)

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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 textbooks/websites

Fundamental Theorem of Calculus using definite integration

  • a and b are called limits
    • a is the lower limit
    • b is the upper limit
  • f’(x) is the derivative of f(x)
  • The value can be positive, zero or negative

Why do I not need to include a constant of integration for definite integration?

Example of the constant of integration cancelling out

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

How do I find a definite integral?

  • STEP 1
    • Give the integral a name (if it does not already have one) 
      • 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
      • Substitute the top limit first
      • Then substitute the bottom limit
      • Subtract the second value from the first

Example of definite integration

What are the special properties of definite integrals?

  • Some of these have been encountered already and some may seem obvious …
    • taking constant factors outside the integral
      • integral subscript a superscript b k straight f left parenthesis x right parenthesis space straight d x equals k integral subscript a superscript b straight f left parenthesis x right parenthesis space straight d x wherespace k is a constant
      • useful when fractional and/or negative values involved
    • integrating term by term
      • space integral subscript a superscript b left square bracket straight f left parenthesis x right parenthesis plus straight g left parenthesis x right parenthesis right square bracket space straight d x equals integral subscript a superscript b straight f left parenthesis x right parenthesis space straight d x plus integral subscript a superscript b straight g left parenthesis x right parenthesis space straight d x 
      • the above works for subtraction of terms/functions too
    • equal upper and lower limits
      • integral subscript a superscript a straight f left parenthesis x right parenthesis space d x equals 0 
      • on evaluating, this would be a value subtracted from itself!
    • swapping limits gives the same, but negative, result
      • integral subscript a superscript b straight f left parenthesis x right parenthesis space straight d x equals negative integral subscript b superscript a straight f left parenthesis x right parenthesis space straight d x 
      • compare 8 subtract 5 say, with 5 subtract 8 …
    • splitting the interval
      • space integral subscript a superscript b straight f left parenthesis x right parenthesis space straight d x equals integral subscript a superscript c straight f left parenthesis x right parenthesis space straight d x plus integral subscript c superscript b straight f left parenthesis x right parenthesis space straight d x wherespace a less or equal than c less or equal than b
      • this is particularly useful for areas under multiple curves or areas under thespace x-axis

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

Example fig1, 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.