First off, my first thank goes to
Albert, for you provided a proof with diagram in this challenge problem. But, like others, I have a really hard time to digest your proof and I must look at the follow-up post by
Opalg to fully decipher the reasoning behind your proof. Still, I am struggling to see why there must be true that the line $BGH$ is a tangent to the circle at the point $G$. I am sorry...
I want to also thank to
Opalg, for your willingness to answer to
johng's posts, because I always think it's the OP's responsibility to answer to anyone who has found the proof confounding...
My solution:
View attachment 1834
Ceva's theorem tells us the product of the ratios of the pairs of segments formed on each side of the triangle by the intersection point is equal to 1.
Hence we have
$\dfrac{\sin 13^{\circ}}{\sin 13^{\circ}} \cdot \dfrac{\sin 73^{\circ}}{\sin 30^{\circ}} \cdot \dfrac{\sin x}{\sin (51^{\circ}-x)}=1$
$\sin 73^{\circ}\cdot \sin x=\sin 30^{\circ} \cdot \sin (51^{\circ}-x)$
$\sin 73^{\circ}\cdot \sin x=\sin 30^{\circ} \cdot (\sin 51^{\circ} \cos x-\cos 51^{\circ} \sin x)$
$\sin x(\sin 73^{\circ}+\sin 30^{\circ}\cos 51^{\circ})=\sin 30^{\circ} \sin 51^{\circ} \cos x $
$\begin{align*}\tan x&=\dfrac{\sin 30^{\circ} \sin 51^{\circ}}{\sin 73^{\circ}+\sin 30^{\circ}\cos 51^{\circ}}\\&=\dfrac{\dfrac{1}{2} \sin 51^{\circ}}{\sin 73^{\circ}+\dfrac{1}{2}\cos 51^{\circ}}\\&=\dfrac{\sin 51^{\circ}}{2\sin 73^{\circ}+\cos 51^{\circ}}\\&=\dfrac{\sin 3(17)^{\circ}}{2\sin (90-17)^{\circ}+\cos 3(17)^{\circ}}\\&=\dfrac{\sin 17^{\circ}(3-4\sin^2 17^{\circ})}{2\cos 17^{\circ}+\cos 17^{\circ}(4\cos^2 17^{\circ}-3)}\\&=\dfrac{\sin 17^{\circ}(3-4\sin^2 17^{\circ})}{\cos 17^{\circ}(2+4\cos^2 17^{\circ}-3)}\\&=\dfrac{\sin 17^{\circ}(3-4\sin^2 17^{\circ})}{\cos 17^{\circ}(4(1-\sin^2 17^{\circ})-1)}\\&=\tan 17^{\circ}\end{align*}$
$\therefore x=17^{\circ}$