Inverse Functions & Newton's Method: Debmnzl's Question | Yahoo! Answers

In summary, the OP is asking for help with a question about a math function. They found the inverse of the function and it is e^x/a. They need to find a value of $a$ such that the function and its inverse touch only once. They use Newton's method to find the value of $a$ and it is approximately (2.156,2.156).
  • #1
MarkFL
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Here is the question:

A math function question please help?

F(x) = a loge(x) + 1/2
Find the inverse which I found is e^x/a

It then said let a=2 and sketch graph which I did.

I'm getting trouble at this part

Find the value of a in 3 decimal places such that the graphs of y=f(x) and its inverse touch each other once only.

And hence find the contact point.

Please help explanations too please

Here is a link to the question:

A math function question please help? - Yahoo! Answers

I have posted a link there to this topic so that the OP may find my response.
 
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  • #2
Hello Debmnzl,

We are given the function:

$\displaystyle f(x)=a\ln(x)+\frac{1}{2}$

and are asked to compute its inverse. So, we may begin by switching $x$ and $f(x)$, and then solving for $f(x)$ which will actually be $\displaystyle f^{-1}(x)$

$\displaystyle x=a\ln(f(x))+\frac{1}{2}$

Subtract through by $\displaystyle \frac{1}{2}$

$\displaystyle x-\frac{1}{2}=a\ln(f(x))$

Get common denominator on the left:

$\displaystyle \frac{2x-1}{2}=a\ln(f(x))$

Divide through by $a$:

$\displaystyle \frac{2x-1}{2a}=\ln(f(x))$

Convert from logarithmic to exponential form:

$\displaystyle e^{\frac{2x-1}{2a}}=f(x)$

and so we have:

$\displaystyle f^{-1}(x)=e^{\frac{2x-1}{2a}}$

Now, as a means of checking our work, we may use the fact that we require:

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

i) $\displaystyle f\left(f^{-1}(x) \right)=a\ln\left(e^{\frac{2x-1}{2a}} \right)+\frac{1}{2}=a\left(\frac{2x-1}{2a} \right)+\frac{1}{2}=x$

ii) $\displaystyle f^{-1}(f(x))=e^{\frac{2\left(a\ln(x)+\frac{1}{2} \right)-1}{2a}}=e^{\frac{2a\ln(x)}{2a}}=e^{\ln(x)}=x$

So, we know the inverse function we found is correct.

Next, we are instructed to let $a=2$ and sketch the graph. I will assume we are to graph both the given function and its inverse:

View attachment 613

Now, to find a value of $a$ such that the function and its inverse touch only once, we may use the fact that the two are reflected about the line $y=x$, and so must be tangent to this line.

The gradient of the line $y=x$ is 1, and so, we require:

$\displaystyle \frac{d}{dx}\left(f(x) \right)=\frac{a}{x}=1\,\therefore\,a=x$

$\displaystyle \frac{d}{dx}\left(f^{-1}(x) \right)=\frac{e^{\frac{2x-1}{2a}}}{a}=1\,\therefore\,a=e^{\frac{2x-1}{2a}}$

Now since we found $a=x$ we may write:

$\displaystyle a=e^{\frac{2a-1}{2a}}$

To compute the solution to this equation to 3 decimal places, we may use Newton's method. Let's define:

$\displaystyle g(a)=e^{\frac{2a-1}{2a}}-a=0$

where:

$\displaystyle g'(a)=\frac{1}{2a^2}e^{\frac{2a-1}{2a}}-1$

Newton's method gives us the recursion:

$\displaystyle a_{n+1}=a_n-\frac{g(a_n)}{g'(a_n)}$

Using the function we defined and its derivative, we have:

$\displaystyle a_{n+1}=a_n-\frac{e^{\frac{2a_n-1}{2a_n}}-a_n}{\frac{1}{2a_n^2}e^{\frac{2a_n-1}{2a_n}}-1}=\frac{a_n(1-2a_n)}{1-2a^2e^{\frac{1-2a_n}{2a_n}}}$

Using $a_0=2$ as our initial estimate, we then find:

$a_1\approx2.15910252177$

$a_2\approx2.15553677378$

$a_3\approx2.1555352035$

$a_4\approx2.1555352035$

Now, since we were only asked to find the estimate to 3 decimal places, we would write:

$a\approx2.156$

and since we found $a=x$ then the $y$-coordinate of the point of intersection must also be $a$ since the function and its inverse will touch along the line $y=x$

And so the point of intersection is approximately (2.156,2.156). Here is a graph using the better approximation we found:

View attachment 614
 

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FAQ: Inverse Functions & Newton's Method: Debmnzl's Question | Yahoo! Answers

What are inverse functions and how are they related to Newton's method?

Inverse functions are functions that "undo" each other. This means that if a function f(x) maps a certain input x to an output y, then its inverse function f^-1(y) maps the output y back to the input x. Newton's method is a mathematical technique used to find the root of a function, or the value of x that makes the function equal to 0. Inverse functions and Newton's method are related because Newton's method can be used to find the inverse of a function.

How do you find the inverse of a function?

To find the inverse of a function, you need to switch the roles of x and y. This means that for every x-value in the original function, you will have a corresponding y-value in the inverse function. You can then solve for y and rewrite the function in terms of y. The resulting function will be the inverse of the original function.

What is the process for using Newton's method to find the root of a function?

The process for using Newton's method to find the root of a function involves first choosing an initial guess for the root, denoted as x0. Then, you use the formula x1 = x0 - f(x0)/f'(x0) to find a new guess x1. This process is repeated until the value of x converges to the root, or the value of x where the function is equal to 0.

Can Newton's method be used to find the inverse of any function?

No, Newton's method can only be used to find the inverse of functions that are one-to-one (meaning that each input has a unique output) and continuously differentiable (meaning that the slope of the function is defined at every point). If a function does not meet these criteria, Newton's method may not converge or may give incorrect results.

Are there any limitations to using Newton's method?

Newton's method has a few limitations. Firstly, as mentioned before, it can only be used on certain types of functions. Additionally, it may not converge or may give incorrect results if the initial guess for the root is not close enough to the actual root. Lastly, for some functions, Newton's method may converge to a local minimum or maximum instead of the root, resulting in an incorrect solution.

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