Inverse of a function

dttrapo

New member
Joined
Feb 14, 2021
Messages
5
Hello! I'm really struggling with this problem:


Find the inverse of g(x)=−2ln(7x+1)

I'm not sure which steps to take to figure out the inverse here. Any advice would be greatly appreciated!
 

Harry_the_cat

Senior Member
Joined
Mar 16, 2016
Messages
2,444
Let y = -2ln(7x+1)

Rearrange to get x = .... Can you do this step?


Interchange x and y.

Your new y will be the inverse of g(x).
 

HallsofIvy

Elite Member
Joined
Jan 27, 2012
Messages
7,176
Harry_the_cat suggests you solve for x, then swap x and y.
I tend to think "the other way around"- first swap x and y-
y= -2ln(7x+ 1) becomes x= -2ln(7y+1)
then solve for y. Of course it is exactly the same thing.

You "solve for" a variable by "undoing" whatever has been done to it. Here, the equation, x= -2 ln(7y+1), says that, given a value of y, to find x, we would
1) multiply by 7
2) add 1
3) take the logarithm
4) multiply by -2.

To "undo" we do the opposite of each step in the opposite order.
1) the opposite of "multiply by -2" is "divide by -2"
2) the opposite of "take the logarithm" is "take the exponential"
3) the opposite of "add 1" is "subtract 1"
4) the opposite of "multiply by 7" is "divide by 7".
And, of course, whatever we do to one side of the equation we must do to the other side.

So, starting with x= -2 ln(7y+1) we
1) divide by -2: x/(-2)= -x/2= ln(7y+ 1)
2) take the exponential: e^(-x/2)= 7y+ 1
3) subtract 1: e^(-x/2)- 1= 7y
4) divide by 7: (e^(-x/2)- 1)/7= y

g^-1(x)= (e^(-x/2)- 1)/7.

Perhaps it was the logarithm that was bothering you. Remember that the logarithm is defined as the inverse function to the exponential function, e^(ln(x))= x, and so the exponential function is the inverse function to the logarithm: ln(e^x)= x.
 

dttrapo

New member
Joined
Feb 14, 2021
Messages
5
It was definitely the logarithm that was tripping me up. I really struggle with them for some reason. I really appreciate all the help!
 
Top