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its vertices do not send any diagonal

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by sanju09 » Tue May 24, 2011 1:56 am
How many diagonals does a polygon with 18 sides have if three of its vertices do not send any diagonal? Use the formula: number of diagonals = n (n - 3)/2 where n is the number of sides. Each vertex sends of n - 3 diagonals.
(A) 90
(B) 126
(C) 210
(D) 264
(E) 306


[spoiler]Source: Eric's collection on BTG


OA A[/spoiler]
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by Frankenstein » Tue May 24, 2011 3:34 am
Hi,
Three vertices of 18-sided polygon are not sending any diagonals. So, effectively there are only 15 vertices sending diagonals. So, the number of diagonals =15.(15-3)/2 = 90

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