Newton-Raphson Method for derivation of iteration formula

In summary, the conversation discusses the general equation for finding Xn+1 and its counterpart for Xn-1. It also mentions the use of the Newton-Raphson method for finding the iteration of Xn-1 when x1 is any initial estimate for sqrt(a).
  • #1
KataKoniK
1,347
0
Hi,

Prolly a dumb question, but the general equation for finding Xn+1 is

Xn+1 = Xn - f(Xn) / f`(Xn)

What will it be for Xn-1?
 
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  • #2
X(n - 1) = X(n - 2) - f(X(n - 2)) / f'(X(n - 2)) (obviously only valid for n > 2 or n >= 2).
 
  • #3
Or, if you want to "walk backwards" from xm to x(m-1), you can substitute n=m-1 in your equation and solve for x(m-1):

x((m-1)+1) = x(m-1) - f(x(m-1))/f`(x(m-1))
x(m) = x(m-1) - f(x(m-1))/f`(x(m-1))
x(m-1) = x(m) + f(x(m-1))/f`(x(m-1))

From here I'm not quite sure, since you don't have f(x(m-1)) and its derivative,... maybe we can approximate them using the value at m, like so:

x(m-1) = x(m) + f(x(m))/f`(x(m))

But I'm not certain that's a valid approx... is this reasonable, guys?
 
  • #4
Thanks for the help.

The question I am doing is the following.

f(x) = x^2 - a

Show that for x1 > 0 is any initial estimate for sqrt(a) then the Newton-Raphson method gives the iteration

Xn-1 = (1/2)(Xn + a/Xn) , n >= 1
 

1. What is the Newton-Raphson Method?

The Newton-Raphson Method is an iterative method used to find the roots of a given equation. It is commonly used in numerical analysis and is named after its inventors, Sir Isaac Newton and Joseph Raphson.

2. How does the Newton-Raphson Method work?

The method involves using an initial guess for the root and then repeatedly using an iteration formula to refine the guess until the desired level of accuracy is achieved. The iteration formula is based on the tangent line to the curve at the current guess for the root.

3. What is the equation for the Newton-Raphson iteration formula?

The iteration formula is given by: xn+1 = xn - f(xn) / f'(xn), where xn is the current guess for the root, f(xn) is the value of the function at xn, and f'(xn) is the derivative of the function at xn.

4. What are the advantages of using the Newton-Raphson Method?

The Newton-Raphson Method is a very efficient method for finding roots of equations. It typically converges faster than other methods and can work with a wide range of functions. It also allows for a high level of accuracy to be achieved with relatively few iterations.

5. Are there any limitations to using the Newton-Raphson Method?

Yes, there are some limitations to this method. It may fail to converge if the initial guess is too far from the actual root or if the function has multiple roots. It also requires the calculation of the derivative, which can be difficult for some functions.

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