Logarithmic Functions (Edexcel International A Level Maths: Pure 2)

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Logarithmic Functions

Logarithmic functions

Logarithmic Functions Notes fig1, A Level & AS Maths: Pure revision notes

 

  • a = bx and log b a = x are equivalent statements
  • a > 0
  • b is called the base
  • Every time you write a logarithm statement say to yourself what it means
    • log3 81 = 4

      “the power you raise 3 to, to get 81, is 4”

    • logp q = r

      “the power you raise p to, to get q, is r

 

Logarithm rules

 

  • As a logarithm is the inverse of raising to a power

    Logarithmic Functions Notes fig2, A Level & AS Maths: Pure revision notes

How do I use logarithms?

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

 

  • Recognising the rules of logarithms allows expressions to be simplified

 

Logarithmic Functions Notes fig4, A Level & AS Maths: Pure revision notes

  • Recognition of common powers helps in simple cases
    • Powers of 2: 20 = 1, 21 = 2, 22 = 4, 23 = 8, 24 =16, …
    • Powers of 3: 30 = 1, 31 = 3, 32 = 9, 33 = 27, 34 = 81, …
    • The first few powers of 4, 5 and 10 should also be familiar

  • For more awkward cases a calculator is needed

 Logarithmic Functions Notes fig5, A Level & AS Level Pure Maths Revision Notes 

  • Calculators can have, possibly,  different logarithm buttons

 Logarithmic Functions Notes fig6, A Level & AS Maths: Pure revision notes 

  • This button allows you to type in any number for the base

 Logarithmic Functions Notes fig8, A Level & AS Maths: Pure revision notes

  • Shortcut for base 10 although SHIFT button needed 
  • Before calculators, logarithmic values had to be looked up in printed tables

Notation 

  • 10 is a common base
    • log10 x is abbreviated to log x or lg x

  • (log x)2 ≠ log x2

Worked example

Logarithmic Functions Example fig1, A Level & AS Maths: Pure revision notes

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"ln"

What is ln? 

  • ln is a function that stands for natural logarithm
  • It is a logarithm where the base is the constant "e"
    • ln space x identical to log subscript straight e x
    • It is important to remember that ln is a function and not a number

What are the properties of ln? 

  • Using the definition of a logarithm you can see
    • ln space 1 equals 0
    • ln space straight e equals 1
    • ln space straight e to the power of x equals x 
    • ln space x is only defined for positive x
  • As ln is a logarithm you can use the laws of logarithms
    • ln space a plus ln space b equals ln left parenthesis a b right parenthesis
    • ln space a minus ln space b equals ln stretchy left parenthesis a over b stretchy right parenthesis
    • n space ln space a equals ln left parenthesis a to the power of n right parenthesis

How can I solve equations involving e & ln? 

  • The functions straight e to the power of x and ln space x are inverses of each other
    • If straight e to the power of straight f left parenthesis x right parenthesis end exponent equals straight g left parenthesis x right parenthesis then straight f stretchy left parenthesis x stretchy right parenthesis equals ln invisible function application space straight g stretchy left parenthesis x stretchy right parenthesis
    • If ln invisible function application space straight f left parenthesis x right parenthesis equals straight g open parentheses x close parentheses then straight f open parentheses x close parentheses equals straight e to the power of straight g open parentheses x close parentheses end exponent
  • If your equation involves "e" then try to get all the "e" terms on one side
    • If "e" terms are multiplied, you can add the powers
      • straight e to the power of x cross times straight e to the power of y equals straight e to the power of x plus y end exponent 
      • You can then apply ln to both sides of the equation
    • If "e" terms are added, try transforming the equation with a substitution
      • For example: If y equals straight e to the power of x then straight e to the power of 4 x end exponent equals y to the power of 4
      • You can then solve the resulting equation (usually a quadratic)
      • Once you solve for y then solve for x using the substitution formula
  • If your equation involves "ln", try to combine all "ln" terms together
    • Use the laws of logarithms to combine terms into a single term
    • If you have ln invisible function application space straight f open parentheses x close parentheses equals ln invisible function application space straight g left parenthesis x right parenthesis then solve straight f open parentheses x close parentheses equals straight g left parenthesis x right parenthesis
    • If you have ln invisible function application space straight f open parentheses x close parentheses equals k then solve straight f open parentheses x close parentheses equals straight e to the power of k

Worked example

3-1-1-ln-we-solution

Examiner Tip

  • Always simplify your answer if you can
    • for example, 1 half ln space 25 space equals space ln space square root of 25 equals ln space 5
    • you wouldn't leave your final answer as square root of 25 so don't leave your final answer as 1 half ln space 25

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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.