MHB Determine the position using an iteration method

  • Thread starter Thread starter mathmari
  • Start date Start date
  • Tags Tags
    Method Position
AI Thread Summary
The function f(x) has a unique position \overline{x} greater than 1 where it equals f(1). The calculation of f(1) results in \frac{\pi}{e^{1/9}}. To find \overline{x}, an iteration method is suggested, with Newton's method being a viable option. Starting with an initial guess of x_0=1.5, the method converges to approximately 1.41 after several iterations. The final result indicates that the position \overline{x} is 1.41, confirming the accuracy of the calculations.
mathmari
Gold Member
MHB
Messages
4,984
Reaction score
7
Hey! :o

The function \begin{equation*}f(x)=\frac{x}{e^{x/9}}\cdot \frac{\sin \left (\pi (x-1)\right )}{x-1}\end{equation*} has at exactly one position $\overline{x}>1$ the same value as at the position $x=1$. Determine the position $\overline{x}$ using an iteration method with accuracy of two decimal digits. I have done the following:

We have that \begin{equation*}f(\overline{x})=f(1)\Rightarrow f(\overline{x})-f(1)=0 \Rightarrow g(x):=f(x)-f(1)\end{equation*}

First we have to calculate $f(1)$:
\begin{align*}f(1)&=\lim_{x\rightarrow 1}f(x)=\lim_{x\rightarrow 1}\frac{x}{e^{x/9}}\cdot \frac{\sin \left (\pi (x-1)\right )}{x-1}=\frac{1}{e^{1/9}}\cdot \lim_{x\rightarrow 1}\frac{\sin \left (\pi (x-1)\right )}{x-1}\ \overset{DLH}{ = } \ \frac{1}{e^{1/9}}\cdot \lim_{x\rightarrow 1}\frac{\pi \cos \left (\pi (x-1)\right )}{1}\\ & =\frac{1}{e^{1/9}}\cdot \pi \cos \left (\pi \cdot 0\right )=\frac{1}{e^{1/9}}\cdot \pi =\frac{\pi}{e^{1/9}}\end{align*}

Therefore we get the function \begin{equation*}g(x)=\frac{x}{e^{x/9}}\cdot \frac{\sin \left (\pi (x-1)\right )}{x-1}-\frac{\pi}{e^{1/9}}\end{equation*}

Now we have to apply an iteration method to approximate the root of that function, right? Do we use the Newton's method? (Wondering)

We don't have an interval to which the root will belong, we only know that it is greater than $1$. So do we have to guess such an interval to calculate the first input $x_0$? (Wondering)
 
Mathematics news on Phys.org
mathmari said:
Hey! :o

The function \begin{equation*}f(x)=\frac{x}{e^{x/9}}\cdot \frac{\sin \left (\pi (x-1)\right )}{x-1}\end{equation*} has at exactly one position $\overline{x}>1$ the same value as at the position $x=1$. Determine the position $\overline{x}$ using an iteration method with accuracy of two decimal digits. I have done the following:

We have that \begin{equation*}f(\overline{x})=f(1)\Rightarrow f(\overline{x})-f(1)=0 \Rightarrow g(x):=f(x)-f(1)\end{equation*}

First we have to calculate $f(1)$:
\begin{align*}f(1)&=\lim_{x\rightarrow 1}f(x)=\lim_{x\rightarrow 1}\frac{x}{e^{x/9}}\cdot \frac{\sin \left (\pi (x-1)\right )}{x-1}=\frac{1}{e^{1/9}}\cdot \lim_{x\rightarrow 1}\frac{\sin \left (\pi (x-1)\right )}{x-1}\ \overset{DLH}{ = } \ \frac{1}{e^{1/9}}\cdot \lim_{x\rightarrow 1}\frac{\pi \cos \left (\pi (x-1)\right )}{1}\\ & =\frac{1}{e^{1/9}}\cdot \pi \cos \left (\pi \cdot 0\right )=\frac{1}{e^{1/9}}\cdot \pi =\frac{\pi}{e^{1/9}}\end{align*}

Therefore we get the function \begin{equation*}g(x)=\frac{x}{e^{x/9}}\cdot \frac{\sin \left (\pi (x-1)\right )}{x-1}-\frac{\pi}{e^{1/9}}\end{equation*}

Now we have to apply an iteration method to approximate the root of that function, right? Do we use the Newton's method? (Wondering)

We don't have an interval to which the root will belong, we only know that it is greater than $1$. So do we have to guess such an interval to calculate the first input $x_0$? (Wondering)

[DESMOS=-1,4,-1,4]\frac{\left(x\sin\left(\pi\left(x-1\right)\right)\right)}{e^{x/9}(x-1)}[/DESMOS]
The above graph shows that the function repeats its value at $x=1$ when $x$ is somewhere near $1.4$ or $1.5$. So I would take $x_0=1.5$.
 
Last edited:
mathmari said:
Now we have to apply an iteration method to approximate the root of that function, right? Do we use the Newton's method?

We don't have an interval to which the root will belong, we only know that it is greater than $1$. So do we have to guess such an interval to calculate the first input $x_0$?

Hey mathmari!

This is an example where Newton-Raphson can have problems if we are not careful.
If we pick a starting value that is too far from the zero, it will likely not converge.
However, a starting value that starts slightly to the right of the zero, such as the 1.5 that Opalg pointed out, should do the job. And it will converge quadratically.
Starting below 1.4 or above 2.1 will likely diverge though. (Wondering)

Alternatively algorithms are bisection and regula falsi.
First we might search for values that are on opposite sides of the x-axis.
That is, we can start with some initial interval, and then either double or half its size until we find both a positive and a negative function value.
The root must then in between those, after which both bisection and regula falsi will find it. (Thinking)
 
We have \begin{align*}&g(x)=\frac{x\sin \left (\pi(x-1)\right )}{e^{x/9}(x-1)}-\frac{\pi}{e^{1/9}}\\ &g'(x)=\frac{\left [\sin \left (\pi (x-1)\right )+x\pi \cos \left (\pi (x-1)\right )\right ]\left (x-1\right )-x\sin \left (\pi (x-1)\right ) \frac{8+x}{9}}{e^{x/9}(x-1)^2}\end{align*}

Choosing as initial value $x_0=1.5$ we get the following:
\begin{align*}x_1=x_0-\frac{g(x_0)}{g'(x_0)}\approx 1.4259 \\ x_2=x_1-\frac{g(x_1)}{g'(x_1)}\approx 1.4149 \\ x_3=x_2-\frac{g(x_2)}{g'(x_2)}\approx 1.4147\end{align*}
The first two decimal digits are the same as in the previous step and so position that we are looking for is $1.41$. Is everything correct? (Wondering)
 
Yep. (Nod)
 
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
Fermat's Last Theorem has long been one of the most famous mathematical problems, and is now one of the most famous theorems. It simply states that the equation $$ a^n+b^n=c^n $$ has no solutions with positive integers if ##n>2.## It was named after Pierre de Fermat (1607-1665). The problem itself stems from the book Arithmetica by Diophantus of Alexandria. It gained popularity because Fermat noted in his copy "Cubum autem in duos cubos, aut quadratoquadratum in duos quadratoquadratos, et...
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...

Similar threads

Replies
11
Views
2K
Replies
4
Views
1K
Replies
5
Views
1K
Replies
1
Views
1K
Back
Top