A method to compute roots other than sqrt.

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In summary, there are methods for computing roots other than the square root, such as Newton's method and using logarithms and Newton's binomial expansion. These methods involve using an arbitrary value and repeatedly plugging it into a formula to obtain the desired root.
  • #1
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is there a method to compute roots other than sqrt?, like 10th root or 13th root of a number?

and, what are they?
 
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  • #2
loop quantum gravity said:
is there a method to compute roots other than sqrt?, like 10th root or 13th root of a number?

and, what are they?
This one is a good candidate for Newton's method. We choose an arbitrary value x0, then use:
[tex]x_{n + 1} = x_n - \frac{f(x_{n})}{f'(x_{n})}[/tex].
And then let n increases without bound to obtain the answer.
[tex]x = \lim_{n \rightarrow \infty} x_n[/tex].
For example:
Find [tex]\sqrt[3]{4}[/tex]
Let [tex]x = \sqrt[3]{4} \Rightarrow x ^ 3 = 4 \Rightarrow x ^ 3 - 4 = 0[/tex]
We then define f(x) := x3 - 4.
Say, we choose x0 = 1, plug everything into a calculator, use the formula:
[tex]x_{n + 1} = x_n - \frac{x_{n} ^ 3 - 4}{3 x_{n} ^ 2}[/tex].
and we'll have:
x1 = 2
x2 = 1.6666667
x3 = 1.5911111
x4 = 1.5874097
x5 = 1.5874010
x6 = 1.5874010
...
So the value of xn will converge quite fast to [tex]\sqrt[3]{4}[/tex], as n tends to infinity.
 
  • #3
Of course, the most obvious question is 'what is this method you have for computing arbitrary square roots'
 
  • #4
FWIW - Logarithms work well for this. Especially if you're a programmer, and are happy with the inherent imprecision of floating point numbers.

If you take
result = (log(x) / n)

and then convert the result back ie.,

nth_root = exp(result)

you can generate all roots of x.
 
  • #5
If you're doing fractions then you can use Newton's binomial expansion

For instance, work out the fifth root of 31 by expanding [tex](1+x)^{\frac{1}{5}}[/tex] with x = -1/32

[tex](1+x)^{\frac{1}{5}} = 1 + \frac{1}{5}x + \frac{1}{5}\left(-\frac{4}{5}\right)\frac{1}{2!}x^{2} + ...[/tex]

Put in x = -1/32 (which gives excellent convergence) to get

[tex]\left( \frac{31}{32} \right)^{\frac{1}{5}} = 1 + \frac{1}{5}\left(-\frac{1}{32}\right) + \frac{1}{5}\left(-\frac{4}{5}\right)\frac{1}{2!}\left(-\frac{1}{32}\right)^{2} + ... = 1 - \frac{1}{160} - \frac{1}{12800} = \frac{12719}{12800}[/tex]

[tex]\frac{(31)^{\frac{1}{5}}}{2} = \frac{12719}{12800}[/tex]

[tex](31)^{\frac{1}{5}} = \frac{12719}{6400}[/tex]

In decimal form this is 1.9873475. Raise it to the 5th power and get 31.00023361. A nice approximation for 2 minutes work.
 

FAQ: A method to compute roots other than sqrt.

What is the difference between finding the square root and computing roots using a different method?

Finding the square root involves finding the number that, when multiplied by itself, gives the original number. Computing roots using a different method involves finding the number that, when raised to a certain power, gives the original number.

What is the most commonly used method for computing roots other than taking the square root?

The most commonly used method for computing roots other than taking the square root is the Newton-Raphson method, also known as the Newton's method. This is an iterative method that uses an initial guess to approximate the root of a function.

Can the method to compute roots other than the square root be used for any type of number?

Yes, the method to compute roots other than the square root can be used for any type of number, including real and complex numbers. However, the specific method used may vary depending on the type of number being computed.

Is the method to compute roots other than the square root more accurate than taking the square root?

It depends on the specific method being used and the number being computed. In some cases, the method to compute roots other than the square root may be more accurate, while in others, taking the square root may be more accurate.

How can the method to compute roots other than the square root be applied in real-world situations?

The method to compute roots other than the square root can be applied in various fields such as engineering, physics, and finance. It can be used to solve equations and find solutions to problems that involve finding roots of functions.

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