Finding roots of equation using Newtons method

In summary, the conversation is about finding the root of a trigonometric function using Newton's method and the potential issues with convergence. The suggested solution is to choose an initial value that is close to the known solution.
  • #1
andrey21
476
0
1. Hi really struggling with this question any help would be great.
Use Newtons method to find the root of 4sin^2x - x = 0 which lies closest to x=2, correct to 3sf.



Homework Equations





The Attempt at a Solution

 
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  • #2
Jamiey1988 said:
1. Hi really struggling with this question any help would be great.
Use Newtons method to find the root of 4sin^2x - x = 0 which lies closest to x=2, correct to 3sf.



Homework Equations





The Attempt at a Solution


The Newton-Raphson method starts with an initial value [itex]x_0[/itex] and then calculates a sequence [itex]x_1[/itex], [itex]x_2[/itex], ... using the formula
[tex]x_{n+1}= x_n- \frac{f(x_n)}{f'(x_n)}[/tex]

Here [itex]f(x)= 4sin^2(x)- x[/itex] so [itex]f'(x)= 8 sin(x)cos(x)- 1[/itex]

What initial value did you use and what results did you get?
 
  • #3
Well I am at the same point now, however say i choose initial value of 1 when substituting into formula xn -(4sin^2x -x/ 8sinxcosx-1) my calculator is just saying error! is this because of the sin^2 part of the equation?
 
  • #4
Newton's method does not always converge. For example suppose somewhere along the line you get a value of xn that is close to a zero of the derivative function. This will send xn+1 into never-never land. This particular function, [tex]f(x)=4\sin^2(x)-x[/tex], has a very narrow interval of convergence wrt Newton's method. An initial value of 1 is outside that interval.

That said, you should be able to solve this equation without resorting to Newton's method. The convergence interval does contain this known point. Choose an initial value that is close to this known solution.
 

FAQ: Finding roots of equation using Newtons method

What is Newton's method for finding roots of an equation?

Newton's method is an iterative numerical method for approximating the roots of a function. It involves using the derivative of the function to iteratively refine an initial guess until a desired level of accuracy is achieved.

How does Newton's method work?

Newton's method starts with an initial guess, x0, for the root of the equation. It then uses the derivative of the function, f'(x), to calculate the next guess, x1, using the formula x1 = x0 - f(x0)/f'(x0). This process is repeated until the desired level of accuracy is reached.

What are the advantages of using Newton's method?

One advantage of Newton's method is that it can converge to the root of an equation quickly, often in just a few iterations. It is also relatively easy to implement and can handle a wide range of functions.

Are there any limitations to using Newton's method?

Yes, there are some limitations to using Newton's method. It may fail to converge if the initial guess is too far from the actual root or if the function has multiple roots. Additionally, it requires knowledge of the derivative of the function, which may not always be readily available.

Can Newton's method be used for finding complex roots?

Yes, Newton's method can be used to find complex roots of a function. However, it requires the function to be differentiable in the complex plane and the initial guess to be a complex number.

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