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Logarithmic Functions (Edexcel International A Level Maths: Pure 2)
Revision Note
Logarithmic Functions
Logarithmic functions
- 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”
- log3 81 = 4
Logarithm rules
- As a logarithm is the inverse of raising to a power
How do I use logarithms?
- Recognising the rules of logarithms allows expressions to be simplified
- 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
- Calculators can have, possibly, different logarithm buttons
- This button allows you to type in any number for the base
- 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
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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"
- 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
- is only defined for positive x
- As ln is a logarithm you can use the laws of logarithms
How can I solve equations involving e & ln?
- The functions and are inverses of each other
- If then
- If then
- 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
- 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 then
- You can then solve the resulting equation (usually a quadratic)
- Once you solve for y then solve for x using the substitution formula
- If "e" terms are multiplied, you can add the powers
- 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 then solve
- If you have then solve
Worked example
Examiner Tip
- Always simplify your answer if you can
- for example,
- you wouldn't leave your final answer as so don't leave your final answer as
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