Here's a challenge for the brave geometers out there:
- Take any triangle (doesn't matter how weird its shape is).
- Trisect all three angles.
- Extends these Trisectors and find where they meet nside the triange.
The Claim:
Your Task:
Proof
Let $PQR$ be an equilateral triangle. Denote the angle $PQR$ by $pQr$, etc. Choose $L$ outside the triangle $PQR$ such that $$lQr = lRq = \beta + \gamma$$ Clearly $LP$ bisects $qLr$. $M$ and $N$ are similarly defined. Let $LQ$ meet $MP$ in $C$. Then, $$qCp = lQp + mPq - \pi = \alpha + \beta + 2\gamma - \frac{\pi}{3} = \gamma$$ Since $\alpha + \beta + \gamma = \frac{\pi}{3}$ Choose $B$ on $LR$ so that $CP$ bisects $qCb$, $P$ is the in-centre of triangle $LCB$ and so, $$pBr = pBc = \frac{1}{2}(\pi - rLq - lCb) = \beta$$ Giving $bPr + rPn = \pi$. Hence $B$ is the intersection of $LR$ and $NP$. Similarly, if $A$ is the intersection of $MR$ and $NQ$, we have, $$qCa = \gamma; \ qAr = qAc = \alpha$$ $$\text{also,} \ rBa = \beta; \ rAb = \alpha$$ And so $ABC$ is the required triangle, and the theorem is proved.
Reference: Roger Penrose, Collected Works, Volume-1.
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