How many diagonals does a polygon of nine sides have?

(A) 72

(B) 54

(C) 36

(D) 27

(E) 24

## Tuesday, May 5, 2009

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- Ariel Goldberg
- My name is Ariel Goldberg and I have been a GMAT tutor for eight years. I have taken the GMAT more than twenty times and scored into the 99th percentile; I like to share my GMAT knowledge with everybody. One of the things I like is to write GMAT quant questions that do reflect the changes in the test. The questions sold by some prep services are outdated in that they reflect the GMAT of three or four years ago, before Pearson took over. So that is where I come in, I provide people with good, real-looking GMAT questions.

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