Evaluating the inverse of a function

So, while your response is, as usual, excellent, it may not have been what was being asked for.In summary, to find the inverse of the given function f(x) = x^5 + x^3 + x, we can set the function equal to 3 and solve for x. By inspection, we can see that the inverse of f(x) at x=1 is equal to 3, making the solution for f^-1(3) equal to 1.
  • #1
Umar
37
0
If f(x) = x^5 + x^3 + x, find f^-1 (3).

I know we have to set the function equal to 3, and solve for x, but I don't think you can simplify the right side. Any help?
 
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  • #2
I think, if given:

\(\displaystyle f(x)=x^5+x^3+x\)

I would look at:

\(\displaystyle f'(x)=5x^4+3x^2+1=5\left(x^4+\frac{3}{5}x^2\right)+1=5\left(x^4+\frac{3}{5}x^2+\frac{9}{100}\right)+1-\frac{9}{20}=5\left(x^2+\frac{3}{10}\right)^2+\frac{11}{20}\)

We see then that for all $x$, we have $f'>0$ and so $f$ is monotonically increasing, and thus $f(x)+C$ will have only 1 real root. And so:

\(\displaystyle f(x)=3\)

will have only 1 real-valued solution. By inspection we see that:

\(\displaystyle f(1)=3\)

Hence:

\(\displaystyle f^{-1}\left(f(1)\right)=f^{-1}\left(3\right)\)

or:

\(\displaystyle f^{-1}\left(3\right)=1\)
 
  • #3
In general, it is very difficult to find an inverse function but, as MarkFL pointed out, for this particular function, it is easy to see that [tex]f(1)= 1^5+ 1^3+ 1= 3[/tex]. The "hard part" was showing that this function has and inverse but, from your first post, it seems you were not required to do that.
 

FAQ: Evaluating the inverse of a function

1. What is the inverse of a function?

The inverse of a function is a mathematical operation that reverses the output of a given function. In simpler terms, it is a function that "undoes" the action of the original function.

2. Why is it important to evaluate the inverse of a function?

Evaluating the inverse of a function allows us to solve equations that involve the original function. It also allows us to find the input for a given output, which can be useful in real-world applications.

3. How do you find the inverse of a function?

To find the inverse of a function, you can follow these steps:

  • Write the original function as y = f(x).
  • Swap the positions of x and y, so the equation becomes x = f(y).
  • Solve for y to get the inverse function, y = f-1(x).

4. Can any function have an inverse?

No, not all functions have an inverse. For a function to have an inverse, it must be a one-to-one function, meaning that each input has a unique output. If multiple inputs have the same output, the inverse function cannot be defined.

5. How can you check if a function and its inverse are correct?

To check if a function and its inverse are correct, you can use the composition method. Plug in the original function into the inverse function and vice versa. If the results are the same as the input, then you have the correct inverse.

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