# Invertible function

## Homework Statement

Find the smallest value of b so that the function f(x) = x^3 + 9x^2 + bx + 8 is invertible.

## The Attempt at a Solution

I know that the function has to be only increasing/decreasing, and I think it is needed to find the derivative of the function. I do however not know how b should be found.

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PeroK
Homework Helper
Gold Member

## Homework Statement

Find the smallest value of b so that the function f(x) = x^3 + 9x^2 + bx + 8 is invertible.

## The Attempt at a Solution

I know that the function has to be only increasing/decreasing, and I think it is needed to find the derivative of the function. I do however not know how b should be found.
Why not find the derivative and see what you can do with that?

I did find the derivative, i.e

f'(x) = 3x^2 + 18x + b. But I am unsure what to do from there.

PeroK
Homework Helper
Gold Member
I did find the derivative, i.e

f'(x) = 3x^2 + 18x + b. But I am unsure what to do from there.
What do you know about an increasing function and its derivative?

It´s positive, but I am a bit confused re. how I should treat b when x is also unknown.

PeroK
Homework Helper
Gold Member
It´s positive, but I am a bit confused re. how I should treat b when x is also unknown.
Okay, so you need $f'(x)$ to be generally positive. Can you graph the function $f'(x)$ for some values of $b$ to see what's happening? E.g. $b=0, 10, 100$

Okay, so you need $f'(x)$ to be generally positive. Can you graph the function $f'(x)$ for some values of $b$ to see what's happening? E.g. $b=0, 10, 100$
Thanks. For b = 27 the derivative is zero at x = -3, but is positive for all other values of x. Is this the correct answer?

PeroK
Yes, any less than 27 and the derivative is negative on an interval. You might like to think about the invertibility of a function that has an inflection point, as is the case for $b =27$ here.