It is really a Brilliant symbol

Geometry Level 5

A regular 12 12 -gon is inscribed in a circle of radius 12. The sum of the lengths of all sides and diagonals of the 12 12 -gon can be written in the form of p + q 2 + r 3 + s 6 p+q\sqrt{2}+r\sqrt{3}+s\sqrt{6} , where p , q , r p,q,r and s s are positive integers. Find p + q + r + s p+q+r+s .


The answer is 720.

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1 solution

Rishabh Jain
Jan 9, 2016

There are 5 different length diagonals which are numbered 1,2,3,4,5 in the figure.

Type 1 diagonals are only 6 in number while 2,3,4 and 5 type diagonals are 12 each. The angle they subtend at the centre are π , 5 π 6 , 2 π 3 , π 2 , π 3 \pi, \frac{5\pi}{6},\frac{2\pi}{3},\frac{\pi}{2},\frac{\pi}{3} respectively. Using cosine rule their lengths are 2Rsinx, where x is half of the angle these diagonals subtend at the centre. While the length of each side is 2Rsin π 12 and subtend π 6 at the centre. \text{While the length of each side is 2Rsin}\frac{\pi}{12} \text{and subtend }\frac{\pi}{6} \text {at the centre.} Hence required sum= 12 × 2 R ( sin π 12 + sin 5 π 12 + sin π 3 + s i n π 4 + sin π 6 ) + 6 × 2 R sin π 2 12\times 2R(\sin\frac{\pi}{12}+ \sin\frac{5\pi}{12}+\sin\frac{\pi}{3}+sin \frac{\pi}{4}+ \sin\frac{\pi}{6})+6\times 2R\sin\frac{\pi}{2} = 144 ( 2 + 6 + 3 + 2 ) \color{#69047E}{=144(2+\sqrt{6}+\sqrt{3}+\sqrt{2})} p + q + r + s = 144 × 5 = 720 \color{#D61F06}{p+q+r+s=144\times 5=720}

Though my method was the same, yours is a neat clear approach. Up voted.

Niranjan Khanderia - 5 years, 5 months ago

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T h a n k s \Large\color{#69047E}{Thanks}

Rishabh Jain - 5 years, 5 months ago

Nice Solution, I didn't thought that one!

Jess McAllister Alicando - 5 years, 5 months ago

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