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In this article Alex and Neil from
Madras College give a generalisation of the Three By
One problem. See also the article by
the same authors,
8 Methods for 'Three by One' which, as
the title suggests, brilliantly solves the same problem using 8
different topics in mathematics thus exemplifying the unity of the
subject.
Given a rectangle of dimensions 1 by
$n$, for each angle, $\alpha_m$ where $m$ is a positive integer,
are there two other angles $\alpha_p$ and $\alpha_q$ whose sum is
equal to $\alpha_m$?
Now \begin{eqnarray} \alpha_m &
= & \tan^{-1}(1/m)\\ \alpha_p & = & \tan^{-1}(1/p)\\
\alpha_q & = & \tan^{-1}(1/q) \end{eqnarray} Using an
exhaustive search for each $m$ from 1 to 12 the $(p,q)$ which
satisfy this are:
|
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
( p ,
q ) pairs |
(2,3) |
(3,7) |
(4,13) |
(5,21) |
(6,31) |
(7,43) |
(8,57) |
(9,73) |
(10,91) |
(11,111) |
(12,133) |
(13,157) |
(3,2) |
|
(5,8) |
|
(7,18) |
|
(9,32) |
(13,21) |
(11,50) |
|
(13,72) |
|
|
|
|
|
|
|
(12,17) |
|
|
|
|
|
Now, using the tan angle sum formula: \begin{eqnarray}
\alpha_m = \alpha_p + \alpha_q & \Rightarrow
&\tan^{-1}(1/m)= \tan^{-1}(1/p)+ \tan^{-1}(1/q)\\ &
\Rightarrow &\tan(\tan^{-1}(1/m))= \tan(\tan^{-1}(1/p)+
\tan^{-1}(1/q))\\ & \Rightarrow & {1\over m} = {(1/p)+
(1/q)\over 1 - (1/pq)}\\ & \Rightarrow & {1\over m} = {p +
q \over pq - 1}\\ & \Rightarrow & q = {mp + 1 \over p - m}
\end{eqnarray} If $p$ and $q$ satisfy this diophantine equation
then these will be solutions to the above question for
$\alpha_m$.
Conjecture 1:
For all $m$ there exists at least one solution where $p = m +
1$.
Proof:
To find a solution, use: $$q = {mp+1\over p - m}.$$ When $p =
m+1$ $$q = m^2+ m +1.$$ Since $p$ and $q$ are integers there are
two angles in the extended diagram which add up to
$\alpha_m$.