Let us first assume that if we wish to find the approximate root of 13 that this will be 3 with 4 left over. The remainder should be divided by 6 (double the 3 above) which gives ^{2}/_{3} . This is the first fraction to be added to the 3, making 3 ^{2}/_{3} which is the approximate root of 13. Since the square of this number is 13 ^{4}/_{9} , it is ^{4}/_{9} too large, and if one wishes a closer approximation, the 6, which is double the 3, should be added to the fraction ^{2}/_{3} giving 6 ^{2}/_{3} and this number should be divided into 4 which is the difference between 13 and 9 ...
[ 3 + x = √13 ]
Its square is 9 plus 6 unknowns plus 1 power. We set this equal to 13.
[ (3 + x)^{2} = 9 + 6x + x^{2} = 13 ]
Subtracting 9 from either side of the equation we are left with 4 equal to 6 unknowns plus 1 power.
[ 6x + x^{2} = 4 ]
Many people have neglected the power and merely set 6 unknowns equal to 4. The unknown then come out to ^{2}/_{3}
[ 6x = 4 gives x = ^{2}/_{3} ]
and the approximate value of the root is 3 ^{2}/_{3} since it has been set equal to 3 plus 1 unknown.
[ √13 = 3 + x = 3 ^{2}/_{3}]
However, taking the power into account, if the unknown is equal to ^{2}/_{3} the power will be ^{2}/_{3} of an unknown which, added to the 6 unknowns, will give us 6 and ^{2}/_{3} unknowns, which equal 4
[ 6x + x^{2} = 4 implies 6x + ^{2}/_{3} x = 4 ]
so the unknown will be equal to ^{3}/_{5}, and since the approximate is 3 plus 1 unknown, it comes to 3 ^{3}/_{5} .
[ x = 4/(6 + ^{2}/_{3}) = implies 3 + x = 3 ^{3}/_{5} ]
But if the unknown is equal to ^{3}/_{5} the power will be ^{3}/_{5} of an unknown and we obtain 6 ^{3}/_{5} unknowns equal to 4
[ 6x + x^{2} = 4 implies 6x + ^{3}/_{5} x = 4]
Then the unknown comes to ^{20}/_{33} .
[ 6x + ^{3}/_{5} x = 4 implies x = 4/(6 + ^{3}/_{5}) = ^{20}/_{33} ]
... this process can be carried to within an imperceptible difference.
With some modern tools, let us investigate what Bombelli is doing. Here is Maple code for Bombelli's recursive procedure
This gives the first 10 fractions as: ^{2}/_{3} , ^{3}/_{5} , ^{20}/_{33} , ^{66}/_{109} , ^{109}/_{180} , ^{720}/_{1189} , ^{2378}/_{3927} , ^{3927}/_{6485} , ^{25940}/_{42837} , ^{85674}/_{141481} The last term as a decimal correct to 12 places is .605551275436 Compare this with the fractional part of √13: .605551275464 Of course we can also see that Bombelli's procedure leads to a continued fraction expansion of a square root. Cataldi's work on continued fractions appeared in 1613, well after Bombelli's work of 1572. Since Bombelli is not thinking in terms of continued fractions it would not be appropriate to claim that he was the inventor of continued fractions because of the above work. however, it is quite possible that Cataldi had the idea of a continued fraction after reading Bombelli's book. |
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