we have
768 = 1024 * \dfrac{3}{4}= (32 * \dfrac{\sqrt{3}}{2})^2 = (32 *\cos(\dfrac{\pi}{6}))^2
so \sqrt{768} = 32 *\cos(\dfrac{\pi}{6})
so
32 + \sqrt{768} = 32 *(1+\cos(\dfrac{\pi}{6})
= 64 *\cos^2(\dfrac{\pi}{12} using 1+\cos 2x = 2 cos^2 x
so
\sqrt{32 + \sqrt{768}} = 8 \cos(\dfrac{\pi}{12})
so
8 + \sqrt{32 + \sqrt{768}} = 8 ( 1 + \cos(\dfrac{\pi}{12})) = 16 \cos^2 \dfrac{\pi}{24}
so \sqrt{8 + \sqrt{32 + \sqrt{768}}} = 4 \cos \dfrac{\pi}{24}
so x = 4, y = 24 or (x,y) = (4,24)
Note:
I have solved this as a problem of the week at http://mathhelpboards.com/potw-secondary-school-high-school-students-35/problem-week-145-january-5th-2015-a-13891.html
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