9514 1404 393
Answer:
√13 ≈ 3.60555127546
"by hand": 3 4/7 or 398143/110425 or 3.6055 or 3 23/38
Explanation:
Most on-line calculators, calculator apps for phone or tablet, and any spreadsheet can calculate this for you. The first attachment shows the result from the Go.ogle calculator.
Here, we describe 4 methods of approximating the root "by hand." That is, the calculations can be done with pencil and paper. They are easily within the capability of a 4-function calculator.
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Method 1 -- Linear approximation
You can approximate it by hand in the following way.
√13 is between √9 = 3 and √16 = 4. As a first approximation, it is ...
3 + (13 -9)/(16 -9) = 3 4/7 ≈ 3.57 ≈ 3.6
This is a linear interpolation between √9 and √16.
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Method 2 -- Babylonian method
The "Babylonian method" can be used to refine this approximation. For the next guess, average this number and the quotient 13 divided by this number.
x = (3 4/7 + (13/(3 4/7))/2 = (25/7 + 13·7/25)/2 = (625 +637)/350 = 631/175
x = 3 106/175 ≈ 3.6057 . . . . . . good to 3 decimal places
One more iteration will double the number of correct decimal digits.
x = (3 106/175 + 13/(3 106/175))/2 = 398143/110425 ≈ 3.60555127915
(correct digits are highlighted)
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Method 3 -- "long division"
There is also a "long-division" method that develops the root one digit at a time. This is described in the second attachment, an extract from the NIST web site. For a number like 13, you would use 13.00 00 00 00 as the initial "dividend" to get a 4-digit root.
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Method 4 -- continued fraction
Once you realize that the root is between 3 and 4, you can write a "continued fraction" for the fractional part:
3 +x = √13 . . . . . . where x is the fractional part of the root
(3 +x)² = 13 . . . . . . square both sides
x² +6x +9 = 13 . . . expand the square
x(x +6) = 4 . . . . . . . subtract 9 and factor
x = 4/(6 +x) . . . . . . write as an iterator (continued fraction)
At this point, we can say x = p/q for some integers p and q. Then the next iteration is ...
p/q = 4/(6 +p/q) = 4q/(6q +p)
That is, successive approximations of x can be had by the transformation ...
(p, q) ⇒ (4q, 6q+p)
Starting with the linear interpolation fraction x=4/7, the next refinements using this method are ...
(p, q) = (4, 7) ⇒ (28, 46) = (14, 23)* ⇒ (92, 152) = (23, 38)
At this point, you have approximated the root as 3 23/38 ≈ 3.6053, accurate in the third decimal place. Convergence is clearly much slower than using the Babylonian method.
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* We have reduced the fraction p/q = 28/46 to 14/23 in order to keep the numbers as small as possible. Similarly, 92/152 is reduced to 23/38.