Composite Functions (CIE IGCSE Additional Maths)
Revision Note
Author
PaulExpertise
Maths
Composite Functions
What is a composite function?
- A composite function is where one function is applied after another function
- The ‘output’ of one function will be the ‘input’ of the next one
- Sometimes called function-of-a-function
- A composite function can be denoted
- All of these mean “ of ”
How do I work with composite functions?
- Recognise the notation
- means “f of g of x”
- First apply to to get
- Then apply to to get
- Always start with the function closest to the variable
- is not usually equal to
Special cases
- and are generally different but can sometimes be the same
- is written as
- Note that trig functions are exceptions to this rule
- e.g. means not
- Note that trig functions are exceptions to this rule
- For inverse functions,
Worked example
Two functions, and are
a)
Find and .
b)
Find, in terms of , .
is the first function to be applied ...
Domain & Range of Composite Functions
How do I find the domain and range of composite functions?
- Use logic to determine the domain and range of a composite function
- For the first function to be applied will be
- So, at best, the domain of will be the same as the domain of
- However, for this to be the case, the range of must be contained within the domain of
- If this is not the case, then restrictions on the domain of will be required
- Similarly, at best, the range of will be the same as the range of
- But if the domain of has been affected, the range of will also be affected
Exam Tip
- Domain and range are important in composite funcitons like
- the ‘output’ (range) of g must be in the domain of f(x), so could exist,
but may not (or not for some values of )
- the ‘output’ (range) of g must be in the domain of f(x), so could exist,
Worked example
Two functions, and are defined as follows
a)
Write down the range of and the range of .
As the domain of is , will always be greater than 1,
The range of is
The square of any value will be positive or zero, but here, is not included in the domain for .
The range of is
b)
Use your answers to (a) to help explain why does not exist.
is the first function to be applied
The range of would need to be contained within the domain of
But the range of is which is outside the domain of which is
does not exist
c)
Find the range of ,
is the first function. The range of is . This is the same as the domain of .
The range of is
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