MHB Challenge Problem #6: Prove tan 18°=√(1-2/√5)

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The discussion focuses on proving that tan 18° equals √(1 - 2/√5) without using any computational tools. The proof is derived from the geometry of a regular pentagon, where the relationship between the lengths of the sides and diagonals is established. By applying the Pythagorean theorem within specific triangles formed in the pentagon, the relationship is confirmed. The geometric approach highlights the connection between the angles and side lengths, leading to the conclusion that tan 18° can be expressed in the desired form. This proof emphasizes both geometric intuition and analytical reasoning.
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Prove that
$$\tan18^\circ\ =\ \sqrt{1-\dfrac2{\sqrt5}}.$$
No calculator, computer program, Excel, Google, or any other kind of cheating tool allowed. (Smirk)

Have fun!
 
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It's true. Proof: It's intuitively obvious.

-Dan
 
Olinguito said:
Prove that
$$\tan18^\circ\ =\ \sqrt{1-\dfrac2{\sqrt5}}.$$
No calculator, computer program, Excel, Google, or any other kind of cheating tool allowed. (Smirk)

[sp]This comes from the geometry of the pentagon.
[TIKZ][scale=0.75]
\coordinate [label=above: $C$] (C) at (90:5cm) ;
\coordinate [label=above right: $D$] (D) at (18:5cm) ;
\coordinate [label=above left: $B$] (B) at (162:5cm) ;
\coordinate [label=below: $A$] (A) at (234:5cm) ;
\coordinate [label=below: $E$] (E) at (306:5cm) ;
\coordinate [label=below: $P$] (P) at (-4.755,-4.045) ;
\coordinate [label=below: $R$] (R) at (4.755,-4.045) ;
\coordinate [label=above right: $Q$] (Q) at (0.45,0.6) ;
\draw (A) -- node
{$1$} (B) -- node[above left] {$1$} (C) -- node[above right] {$1$} (D) -- node
{$1$} (E) -- node[below] {$1$} (A) -- (P) -- (B) -- (D) -- (R) -- (E) ;
\draw (C) -- (E) -- node[above right] {$d$}(B) ;
\draw (-3.4,-3.75) node {$72^\circ$} ;[/TIKZ]
If $ABCDE$ is a regular pentagon with side $1$ then it is a standard result that the diagonals have length $d = \frac12(\sqrt5+1)$, the golden ratio. A quick way to see that is to notice that in the above diagram the triangles $CQD$ and $BQE$ are similar (with angles $36^\circ$ and $108^\circ$), from which $QD = \frac1d$. Then $BD = BQ+QD$, so that $d = 1 + \frac1d$, a quadratic equation whose positive root is the golden ratio.

If $BP$ and $DR$ are the perpendiculars from $B$ and $D$ to $AE$ then $d = PR = PA + AE + ER = 1+2PA$, from which $PA = \frac12(d-1) = \frac14(\sqrt5-1)$.

By Pythagoras in the triangle $BPA$, $BP^2 = 1 - \frac1{16}(\sqrt5-1)^2 = \frac18(5+\sqrt5)$. Therefore $$\frac{PA^2}{BP^2} = \frac{\frac18(3-\sqrt5)}{\frac18(5+\sqrt5)} = \frac{(3-\sqrt5)(5-\sqrt5)}{(5+\sqrt5)(5-\sqrt5)} = \frac{20-8\sqrt5}{20} = 1 - \frac2{\sqrt5}.$$ But $\frac{PA}{BP} = \tan 18^\circ$, and therefore $\tan 18^\circ = \sqrt{1 - \frac2{\sqrt5}}.$

[/sp]​
 
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Thanks, Opalg! It’s a neat geometric solution. (Clapping)

My own solution is entirely analytic. (Nerd)

Consider $\tan54^\circ$. On the one hand:
$$\tan54^\circ\ =\ \frac1{\tan36^\circ}\ =\ \frac{1-\tan^218^\circ}{2\tan18^\circ}.$$
On the other hand:
$$\tan54^\circ\ =\ \tan3(18)^\circ\ =\ \frac{3\tan18^\circ-\tan^318^\circ}{1-3\tan^218^\circ}$$
using the triple-angle formula for tangent. Hence:
$$\frac{1-\tan^218^\circ}{2\tan18^\circ}\ =\ \frac{3\tan18^\circ-\tan^318^\circ}{1-3\tan^218^\circ}$$
$\implies\ 1-4\tan^218^\circ+3\tan^418^\circ\ =\ 6\tan^218^\circ-2\tan^418^\circ$

$\implies\ 5\tan^418^\circ-10\tan^218^\circ+1\ =\ 0$

$\implies$ $\tan^218^\circ$ is a root of the quadratic equation $f(x)=5x^2-10x+1=0$

$\implies\ \tan^218^\circ=\dfrac{10\pm\sqrt{80}}{10}=1\pm\dfrac2{\sqrt5}.$

But $f(1)=-4<0$ and $f(2)=1>0$ showing that there is a root between $1$ and $2$. This root can’t be $\tan^218^\circ$ as $0<\tan18^\circ<1$. It follows that $\tan^218^\circ$ is the lesser of the two possible roots, i.e.
$$\tan^218^\circ\ =\ 1-\frac2{\sqrt5}$$
and we are done. (Cool) (The other root turns out to be $\tan^254^\circ$.)
 
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