Inverse Functions
What is an inverse function?
- An inverse function does the exact opposite of the function it came from
- For example, if the function “doubles the number and adds 1” then its inverse is
- “subtract 1 and halve the result”
- It is the inverse operations in the reverse order
How do I write inverse functions?
- An inverse function f-1 can be written as or
- For example, if its inverse can be written as
- or
How do I find an inverse function?
- The easiest way to find an inverse function is to 'cheat' and swap the and variables
- Note that this is a useful method but you MUST remember not to do this in any other circumstances in maths
- STEP 1
Write the function in the form
e.g. - STEP 2
Swap the 's and's to get
e.g. - STEP 3
Rearrange the expression to make the subject again - STEP 4
Rewrite using the correct notation for an inverse function- either as f-1(x) = … or f-1 : x ↦ …
- should not exist in the final answer
- e.g.
How does a function relate to its inverse?
- If then the input of 3 gives an output of 10
- The inverse function undoes f(x)
- An input of 10 into the inverse function gives an output of 3
- If then
-
- If you apply a function to x, then immediately apply its inverse function, you get x
- Whatever happened to x gets undone
- f and f-1 cancel each other out when applied together
- If you apply a function to x, then immediately apply its inverse function, you get x
- If and you want to solve
- Finding the inverse function in this case is tricky (impossible if you haven't studied logarithms)
- instead, take f of both sides and use that cancel each other out:
Worked example
Find the inverse of the function .
Write the function in the form and then swap the and
Rearrange the expression to make the subject again.
Rewrite using the correct notation for an inverse function.