William Oughtred: A Great Seventeenth-Century Teacher of Mathematics — John Shaqi
William Oughtred: A Great Seventeenth-Century Teacher of MathematicsCajori, Florian
History
William Oughtred: A Great Seventeenth-Century Teacher of Mathematics
Cajori, Florian
Mathematicians -- Biography; Mathematics -- Study and teaching -- History -- 17th century; Oughtred, William, 1575-1660
2 4 7̇ | 6 5 1̇ | 7̣ 1̣ 3̣̇ | ( 4 1 7
------+-------+-------+------------
4 2 | 0 0 0 | 0 | C_q
------+-------+-------+------------
6 4 | | | A_c
1 6 8 | 0 0 0 | 0 | C_q A
------+-------+-------+------------
2 3 2 | 0 0 0 | 0 | Ablatit.
===================================
R 1 5 | 6 5 1̇ | 7 1 3̣ |
------+-------+-------+------------
4 | 8 | | 3 A_q
| 1 2 | | 3 A
4 | 2 0 0 | 0 0 | C_q
------+-------+-------+------------
9 | 1 2 0 | 0 0 | Divisor.
------+-------+-------+------------
4 | 8 | | 3 A_q E
| 1 2 | | 3 A E_q
| 1 | | E_c
4 | 2 0 0 | 0 0 | C_q E
------+-------+-------+------------
9 | 1 2 1 | 0 0 | Ablatit.
===================================
R 6 | 5 3 0 | 7 1 3̣̇ | 4 | 1 |
------+-------+-------+------------ ----+-----+---
| 5 0 4 | 3 | 3 A_q | |
| 1 | 2 3 | 3 A 1 6 | 8 |
| 4 2 0 | 0 0 0 | C_q | 1 |
------+-------+-------+------------ ----+-----+---
| 9 2 5 | 5 3 0 | Divisor. 1 6 8 1
------+-------+-------+------------
3 | 5 3 0 | 1 | 3 A_q E
| 6 0 | 2 7 | 3 A E_q
| | 3 4 3 | E_c
2 | 9 4 0 | 0 0 0 | C_q E
------+-------+-------+------------
6 | 5 3 0 | 7 1 3 | Ablatit.”
Next, he evaluates the coefficients of E in 3A²E and 420000E, also 3A,
the coefficient of E². He obtains 3A²=480000, 3A=1200, C_q=420000. He
interprets 3A² and C_q as tens, 3A as hundreds. Accordingly, he obtains
as their sum 9120000, which is the divisor for finding the second digit
in the approximation. Observe that this divisor is the value of
|f(a+s₁)-f(a)|-s₁ⁿ in our general expression, where a=400, s₁=10, n=3,
f(x)=x³+420000x.
Dividing the remainder 15651713 by 9120000, he obtains the integer 1 in
ten’s place; thus E=10, approximately. He now computes the terms 3A²E,
3AE² and E³ to be, respectively, 4800000, 120000, 1000. Their sum is
9121000. Subtracting it from the previous remainder, 15651713, leaves the
new remainder, 6530713.
From here on each step is a repetition of the preceding step. The new A
is 410, the new E is to be determined. We have now in closer
approximation, L=A+E. This time we do not subtract A³ and C_qA, because
this subtraction is already affected by the preceding work.
We find the second trial divisor by computing the sum of 3A², 3A and C_q;
that is, the sum of 504300, 1230, 420000, which is 925530. Again, this
divisor can be computed by our general expression for divisors, by taking
a=410, s₁=1, n=3.
Dividing 6530713 by 925530 yields the integer 7. Thus E=7. Computing
3A²E, 3AE², E³ and subtracting their sum, the remainder is 0. Hence 417
is an exact root of the given equation.
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