Q. 68 - Precalculus Enhanced with Graphing Utilities

Question

In Problems 61–72, the function f is one-to-one. Find its inverse and check your answer. 

$f\left(x\right)=\frac{2x-3}{x+4}$

Step-by-Step Solution

Answer

The inverse of f  is $\frac{3+4x}{2-x}.$

Step 1. Given Information

The given function is $f\left(x\right)=\frac{2x-3}{x+4}$

We have to find its inverse and check the answer.

Step 2. Replace f(x) with y and interchange the variables x and y

So, $y=\frac{2x-3}{x+4}$

Interchange the variables

$x=\frac{2y-3}{y+4}$

Step 3. Solve for y

$$\begin{gathered} x=\frac{2y-3}{y+4} \\ x\left(y+4\right)=2y-3 \\ xy+4x=2y-3 \\ 4x+3=2y-xy \\ 4x+3=y\left(2-x\right) \\ \frac{4x+3}{2-x}=y \end{gathered}$$

The inverse is $f^{-1}\left(x\right)=\frac{4x+3}{2-x}.$

Step 4. Check the answer

We can check the answers by $f^{-1}\left(f\left(x\right)\right) and f\left(f^{-1}\left(x\right)\right).$

So,

$$\begin{gathered} f^{-1}\left(f\left(x\right)\right)=f^{-1}\left(\frac{2x-3}{x+4}\right) \\ =\frac{4\left({\displaystyle \frac{2x-3}{x+4}}\right)+3}{2-\left({\displaystyle \frac{2x-3}{x+4}}\right)} \\ =\frac{11x}{11} \\ =x \end{gathered}$$

Thus, $f^{-1}\left(f\left(x\right)\right)=x$

Now,

$$\begin{gathered} f\left(f^{-1}\left(x\right)\right)=f\left(\frac{4x+3}{2-x}\right) \\ =\frac{2\left({\displaystyle \frac{4x+3}{2-x}}\right)-3}{\left({\displaystyle \frac{4x+3}{2-x}}\right)+4} \\ =\frac{11x}{11} \\ =x \end{gathered}$$

Thus, $f\left(f^{-1}\left(x\right)\right)=x$

Hence our result is right.