Is there an interpretation for this trigonometric identity?
A while ago I came across the following identity in an online math forum (of which I don't remember the name): $$\tan\left(\frac{\pi}{11}\right)+4\sin\left(\frac{3\pi}{11}\right)=\sqrt{11}.$$
It is not hard to give a proof by rewriting everything in terms of $\exp(i\pi/11)$ and applying a sequence of manipulations. I am wondering where this identity is coming from. Can somebody think of a geometric interpretation? Of an algebraic one?
Edit: Here's an example of what I mean by an algebraic interpretation: The identity $$\sin\left(\frac{\pi}{7}\right)\cdot\sin\left(\frac{2\pi}{7}\right)\cdot\sin\left(\frac{3\pi}{7}\right)=\frac{\sqrt{7}}{8}$$ expresses the fact that for the Chebyshev polynomial $$T_7(x)=x(64x^6-112x^4+56x^2-7)$$ the product of the roots $\displaystyle \sin\left(\frac{k\pi}{7}\right)$, $1\leq k<7$, of the second factor is equal to the normalized constant term $\displaystyle \frac{7}{64}$.
I could only think of a direct trigonometric interpretation of the identity.
The radius of the circular sector is 1. The measures of the central angles and the lengths of the line segments are:
- The smaller angle: $\pi/11$ rad.
- The bigger angle: $3\pi/11$ rad.
- The red line segment: $\sqrt{11}$.
- The vertical black line segment: $4\sin(3\pi/11)$.
- The vertical light red segment: $\tan(3\pi/11)$.
The red line segment is the hypotenuse of the right triangle whose catheti are the line segment with length $\sqrt{10}$ and the orthogonal unit segment. The $\sqrt{10}$ line segment is the hypotenuse of the right triangle whose catheti are the horizontal line segment with length 3 and the vertical line segment with length 1.
Edited: The angle $\pi/11=2\pi/22$ is not constructible with compass and straightedge (Wikipedia, Constructible polygon ). Therefore the figure is an impossible construction with compass and straightedge only.