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# How many diagonals does a 63-sided convex polygon have?

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How many diagonals does a 63-sided convex polygon have? [#permalink]

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15 Mar 2016, 13:02
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How many diagonals does a 63-sided convex polygon have?

A. 1890
B. 1953
C. 3780
D. 3843
E. 3906
[Reveal] Spoiler: OA

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How many diagonals does a 63-sided convex polygon have? [#permalink]

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15 Mar 2016, 13:37
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Bunuel wrote:
How many diagonals does a 63-sided convex polygon have?

A. 1890
B. 1953
C. 3780
D. 3843
E. 3906

A 63-sided convex polygon has 63 vertices.
If we examine a single vertex, we can see that we can connect it with 60 other vertices to create a diagonal. NOTE: there are 60 options because we can't connect the vertex to ITSELF, and we can't connect it to its ADJACENT vertices, since this would not create a diagonal.

If each of the 63 vertices can be connected with 60 vertices to create a diagonal then...
...the total number of diagonals = (63)(60) = 3780
HOWEVER, before we select answer choice C, we must recognize that we have counted every diagonal TWICE.
For example, we might connect vertex A with vertex F and count that as 1 diagonal, and at the same time we connect vertex F with vertex A and count that as 1 diagonal. Of course these diagonals are the SAME.
To account for counting each diagonal twice, we must divide 3780 by 2 to get: 1890
[Reveal] Spoiler:
A

Cheers,
Brent
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Last edited by GMATPrepNow on 08 Aug 2016, 06:39, edited 1 time in total.

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Re: How many diagonals does a 63-sided convex polygon have? [#permalink]

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12 Jun 2016, 04:00
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Bunuel wrote:
How many diagonals does a 63-sided convex polygon have?

A. 1890
B. 1953
C. 3780
D. 3843
E. 3906

63 sided polygon will have 63 vertices.

No. of lines connecting any two vertices = 63C2

So, no. of diagonals = 63C2 - 63(no. of sides) = 1890

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Re: How many diagonals does a 63-sided convex polygon have? [#permalink]

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12 Jun 2016, 04:28
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Bunuel wrote:
How many diagonals does a 63-sided convex polygon have?

A. 1890
B. 1953
C. 3780
D. 3843
E. 3906

Hi,

the method to find number of diagonals in a polygon is nC2 - n...
nC2 is the number of selecting two vertex, but these will also include the adjoining vertex, which will make the EDGES and not diagonal..
so we subtract EDGES from the total
.....

ans = $$nC2-n = 63C2 - 63 =\frac{63!}{61!2!}- 63 = 63*31 -63 = 63(31-1) = 63 * 30 = 1890$$
A
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Re: How many diagonals does a 63-sided convex polygon have? [#permalink]

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12 Jun 2016, 04:51
Bunuel wrote:
How many diagonals does a 63-sided convex polygon have?

A. 1890
B. 1953
C. 3780
D. 3843
E. 3906

No fo diagonals of an n sided polygon is - $$\frac{n(n − 3)}{2}$$ = $$\frac{(63*60)}{2}$$ => 1890

Hence answer will be (A) 1890
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Re: How many diagonals does a 63-sided convex polygon have? [#permalink]

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24 Jul 2017, 21:41
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Re: How many diagonals does a 63-sided convex polygon have? [#permalink]

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24 Jul 2017, 23:13
Bunuel wrote:
How many diagonals does a 63-sided convex polygon have?

A. 1890
B. 1953
C. 3780
D. 3843
E. 3906

The number of diagonals of a convex polygon = (No. of combination of 2 points of polygon ) - (no. of sides)
63C2 - 63
63 * 62/2 - 63 = 1890

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Re: How many diagonals does a 63-sided convex polygon have? [#permalink]

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08 Aug 2017, 13:58
There is a rule for this:
For any geometric figure, the number of sides is equal to the number of vertexes. Let "n" be that number.

At first, you may think that n times n is the number of diagonals but that is not the case for two reasons.

First, with such procedure, you are counting each line joining two vertexes, including the edges of the figure, which are not diagonals. So, for each vertex, you need to discard two lateral lines. That is n-2. In addition, you need to discard the same vertex you are focusing on. That is n-2-1.

So far, we have n vertexes, each with n-1-2 diagonals coming from it, but that is not the end for a second reason. With n(n-2-1) you are counting each diagonal twice; so divide the number by 2.

The formula is then: n(n-3)/2.

For this question, we have that n=63. Let's operate.

63(63-3)/2 ---> 63*60/2 ---> 63*30 ---> 1890. So A is the answer.

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Re: How many diagonals does a 63-sided convex polygon have?   [#permalink] 08 Aug 2017, 13:58
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