Question

Find the rational approximation of √15.

Answer 1

9514 1404 393

Answer:

  1921/496

Step-by-step explanation:

There are many rational approximations to √15, some better than others.

A linear approximation is often used:

  √15 ≈ 3 +(15-3^2)/(4^2-3^2) = 3 6/7 = 27/7

That can be refined by one iteration of the Babylonian method of determining the root:

  √15 ≈ (27/7 +15/(27/7))/2 = (3 6/7 +3 8/9)/2 = 3 55/63 = 244/63

This value is equivalent to the root rounded to 4 decimal places.

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Another iteration of the Babylonian method gives the approximation ...

  √15 ≈ 119071/30744, equivalent to the root rounded to 9 decimal places.

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The approximation 1921/496 is the best approximation that has a denominator under 1000.

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Additional comment

Successive rational convergents of the continued fraction approximation of √15 can be found as ...

  a'/b' = (3a +15b)/(a +3b)

This method adds approximately one more good decimal place per iteration.

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Successive rational approximations can be found using the Babylonian method (Newton's method iteration) as ...

  a'/b' = (a² +15b²)/(2ab)

This method has "quadratic" convergence. It approximately doubles the number of good decimal places with each iteration.

You can use 3/1, 4/1, or 27/7 to begin either of these iterations.