# Finding inverse functions when there is X^2+x

1. Oct 11, 2013

### applestrudle

1. The problem statement, all variables and given/known data

f(x)= 2x + x^2

2. Relevant equations

3. The attempt at a solution

I don't know how to make x the subject

2. Oct 11, 2013

### Ray Vickson

You need to solve for x in the equation x^2 + 2x = y. This is a simple quadratic equation.

3. Oct 11, 2013

### applestrudle

Okay thank you!

I've got it:

X^2 +2x = y

x^2 +2x +1 = y + 1

(x+1)^2 = y + 1

x = (y+1)^-1/2 -1

f-1(x) = (x+1)^-1/2 -1

:)

Last edited: Oct 11, 2013
4. Oct 11, 2013

### Staff: Mentor

No.
Starting from two lines above, you want to solve for x.
When you do this, don't forget that you need ± in there somewhere.
Note that the equation y = x2 + 2x does not give y as a 1-to-1 function of x, so there is no inverse function, unless you place restrictions on the domain.

5. Oct 12, 2013

### skiller

You have raised to the power -1/2. Where did the negative sign come from?

6. Oct 12, 2013

### applestrudle

Sorry, mistake when I was typing! Thank you for pointing it out