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Tuesday, May 5, 2009
Problem 29 - GMAT Geometry
How many diagonals does a polygon of nine sides have?
This problem is similar to the handshake problem. Consider 9 people standing in circle, how many handshakes are possible if the person can't shake hands with the 2 adjacent to him??
= n(n-3) [n-3, because, he cant shake hand with himself either]
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Answer A
ReplyDelete9c2 - 9 sides = 27 (D)
ReplyDeleteThis problem is similar to the handshake problem.
ReplyDeleteConsider 9 people standing in circle, how many handshakes are possible if the person can't shake hands with the 2 adjacent to him??
= n(n-3) [n-3, because, he cant shake hand with himself either]
now A-B handshake is same as B-A handshake. So,
= n(n-3)/2
Thus, applying this logic here... 9x6/2 = 27
Answer is D.
there is a formula for this. n(n-3) divided by 2
ReplyDeleteso that gives (9 * 6)/2 = 27
Logic: 9 sides; 9 triangles.9*3=27
ReplyDeleteLogic: 9 sides; 9 triangles. 9 is the square of 3, therefore opposite vertices share the same lines.
ReplyDelete9*3=27