# Thread: okay, i keep getting this inverse wrong...

1. ## okay, i keep getting this inverse wrong...

Problem:
Let F(x) = (x+2)/(x+7)

so what is F^(-1) (-7)= ?

2. Originally Posted by lsnyder
Problem:
Let F(x) = (x+2)/(x+7)

so what is F^(-1) (-7)= ?
Hi lsnyder,

$f(x)\Rightarrow y=\frac{x+2}{x+7}$

$f^{-1}(x)\Rightarrow x=\frac{y+2}{y+7}$

Now solve for y. Use this inverse function to find $f^{-1}(-7)$

3. i have solved for y, which i get

y=(2-7x)/x

4. Originally Posted by lsnyder
i have solved for y, which i get

y=(2-7x)/x
Hi isnyder,

Not what I got. Check your algebra again and see if you can come up with the correct denominator. Your numerator is fine.

$x=\frac{y+2}{y+7}$

$x(y+7)=y+2$

Distribute on the left and group your y terms togther on the left. Continue from there.

$xy+7x=y+2$

$xy-y=2-7x$

See? Now factor out a y on the left and then divide.

5. i already did that & got

Y=(2-7x)/x

then if replace the x with -7, i get

Y=(2-7(-7))/ (-7)

Y=51/-7

which is wrong
??? what do i do???

y(x-1)=2-7x

y=(2-7x)/(x-1)

When you factor out the y the -1 still remains. So you almost had it.