A regular -gon is inscribed in a circle of radius 12. The sum of the lengths of all sides and diagonals of the -gon can be written in the form of , where and are positive integers. Find .
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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 , 6 5 π , 3 2 π , 2 π , 3 π 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 1 2 π and subtend 6 π at the centre. Hence required sum= 1 2 × 2 R ( sin 1 2 π + sin 1 2 5 π + sin 3 π + s i n 4 π + sin 6 π ) + 6 × 2 R sin 2 π = 1 4 4 ( 2 + 6 + 3 + 2 ) p + q + r + s = 1 4 4 × 5 = 7 2 0