Newtons Method and Finding the 5th root

In summary, the fifth root of 36 can be solved using the recursion formula xn+1 = xn - f(xn)/f'(xn) where f(x) = x^5 - 36 and f'(x) = 5x^4. Using this formula, the approximate value of the fifth root of 36 is x = 2.2801, accurate to four decimal places.
  • #1
1irishman
243
0

Homework Statement



Find the 5th root of 36 accurate to four decimal places



Homework Equations



xn+1 = xn - f(xn)/f'(xn)



The Attempt at a Solution



First I attempted to write the fifth root of 36 in exponential form as show below:

Let the 5th root of 36 = x
Let f(x) = x^1/5 - 36
So, f'(x) = 1/5x^-4/5 Is this right so far?
 
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  • #2
1irishman said:

Homework Statement



Find the 5th root of 36 accurate to four decimal places



Homework Equations



xn+1 = xn - f(xn)/f'(xn)



The Attempt at a Solution



First I attempted to write the fifth root of 36 in exponential form as show below:

Let the 5th root of 36 = x
Let f(x) = x^1/5 - 36
So, f'(x) = 1/5x^-4/5 Is this right so far?

No. You need to solve x5 - 36 = 0.
 
  • #3
LCKurtz said:
No. You need to solve x5 - 36 = 0.

oh okay, but i am confused because i thought that the fifth root of a number in exponent
form was that number raised to the 1/5. I don't understand how it is x raised to the fifth.
 
  • #4
LCKurtz said:
No. You need to solve x5 - 36 = 0.

1irishman said:
oh okay, but i am confused because i thought that the fifth root of a number in exponent
form was that number raised to the 1/5. I don't understand how it is x raised to the fifth.

If you solved that equation by radicals wouldn't you get [itex]x=36^{\frac 1 5}[/itex]? And isn't that what you are asked to find? So you want the root of that equation.
 
  • #5
oh okay. So then it is like below?

f(2) = 2^5 - 36
= - 4
and

f'(2) = 5(2^4)
= 80
 
  • #6
1irishman said:
oh okay. So then it is like below?

f(2) = 2^5 - 36
= - 4
and

f'(2) = 5(2^4)
= 80

No. Don't you have any worked examples in your text? You mentioned above the recursion formula for xn+1. What do you get for that? You need to use it.
 
  • #7
it's okay, it says that in the text...got it from here. thx.
 

1. What is Newton's Method?

Newton's Method is an algorithm used for finding the roots of a mathematical function. It is a numerical method that uses an initial guess to iteratively approach the root of a function until a desired level of accuracy is reached.

2. How does Newton's Method work?

Newton's Method involves taking the derivative of a function at a given point and using that slope to find the next point on the function. This process is repeated until the function approaches the root, or the value at which the function equals zero.

3. What is the significance of finding the 5th root?

The 5th root of a number is the value that, when multiplied by itself 5 times, equals the original number. It is useful in many mathematical applications, including finding the side lengths of a regular pentagon and solving certain types of equations.

4. How accurate is Newton's Method in finding the 5th root?

The accuracy of Newton's Method in finding the 5th root depends on the initial guess and the complexity of the function. Generally, the more iterations that are performed, the more accurate the result will be.

5. What are the limitations of Newton's Method in finding the 5th root?

Newton's Method may fail to find the 5th root if the function is not well-behaved or if the initial guess is not close enough to the root. It may also converge to a local minimum or maximum instead of the desired root. Additionally, it can be computationally intensive for more complex functions.

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