# Inv.Functions

• Oct 28th 2009, 05:57 AM
Inv.Functions
Ok bit of help please guys - i think i'm just twisting myself in unnecessary knots!

The function: $g(x) = \frac{1}{9}(x - 3)^2 - 4 (6 \leq x \leq 9)$

Is the Domain [6,9] ?

How can i then find the image set of $g^-1$ and its rule?

Thank you :)
• Oct 28th 2009, 06:25 AM
e^(i*pi)
Quote:

Ok bit of help please guys - i think i'm just twisting myself in unnecessary knots!

The function: $g(x) = \frac{1}{9}(x - 3)^2 - 4 (6 \leq x \leq 9)$

Is the Domain [6,9] ?

How can i then find the image set of $g^-1$ and its rule?

Thank you :)

Yes the domain is [6,9].

The range can be found by finding f(6) and f(9).

$f(6) = -3$ and $f(9) = 0$ so the range is [-3,0]

When taking the inverse the the domain becomes the range and the range becomes the domain. Therefore the domain of $g^{-1}(x)$ is [-3,0] and the range is [6,9]

Let $y = g(x)$

$9(y+4) = (x-3)^2$

$\pm 3\sqrt{y+4} = x-3$

$x = 3\pm 3\sqrt{y+4}$.

$g^{-1}(x) = 3\pm 3\sqrt{x+4}$

To determine the sign find g^{-1}(0) = 9 = 3+3\sqrt{4} = 9. So we want the positive root:

$g^{-1}(x) = 3+3\sqrt{x+4}$

Domain: [-3,0]
Range : [6,9]
• Oct 28th 2009, 06:47 AM
So is range the same as image set?
• Oct 29th 2009, 01:04 PM
Just to check in my calculator...graphing

$f(x) = \frac{1}{9}(x - 3)^2 - 4$ $(6 \leq x \leq 9)$

this is a parabola, so the inverse function, how should this look as when i put in

$g^{-1}(x) = 3+3\sqrt{x+4}$

why doesnt it look like the reverse?
• Oct 29th 2009, 01:15 PM
corleone2463
if u have the graph of the function then just consider ur y axxis as xaxis and viceversa (i.e rotate ur copy) and now see the graph thats how it wud luk like
• Oct 29th 2009, 01:18 PM
Thats what i thought - so is e^(i*pi) answer wrong?
• Oct 29th 2009, 01:20 PM
corleone2463
answer to wat ....e^(i*pi) is equal to -1
• Oct 29th 2009, 01:28 PM
yes?
• Oct 29th 2009, 01:31 PM
Plato
Quote:

Thats what i thought - so is e^(i*pi) answer wrong?

No, his/her second answer is correct.
$g^{-1}(x)=3\sqrt{x+4}+3$

The range of $g$ is $[-3,0]$ so that is the domain of $g^{-1}$
• Oct 30th 2009, 05:45 AM
HallsofIvy
Quote:

Originally Posted by corleone2463
answer to wat ....e^(i*pi) is equal to -1

I think he meant the person using the forumname $e^{i\pi}$!(Rofl)