Calculating Diagonals of a Polygon: What are the Values of p and q?

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In summary, for a polygon with n sides, the total number of diagonals can be calculated using the formula 1/p . n . (n-q), where p and q are integers. To find the values of p and q, one can reason by considering the number of diagonals from one corner of the polygon, which is n-3, and then multiplying it by the total number of corners (n). This gives n(n-3) diagonals, but each diagonal connects two corners, so the final formula becomes 1/p . n . (n-q). By solving for different values of n, one can determine that p=2 and q=3. This can also be proven by reasoning how many diagonals there are
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Homework Statement


A polygon with n sides has a total of 1/p . n . (n-q) diagonals, where p and q are integers.

(i) Find the values of p and q.

Homework Equations

The Attempt at a Solution


Can someone just help me to start on this? I know that q = 3 and p = 2 but how?
 
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  • #2
Natasha1 said:

Homework Statement


A polygon with n sides has a total of 1/p . n . (n-q) diagonals, where p and q are integers.

(i) Find the values of p and q.

Homework Equations

The Attempt at a Solution


Can someone just help me to start on this? I know that q = 3 and p = 2 but how?
Take a polygon with n=10 (for example). How many diagonals can you draw from one of the corners?
 
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Natasha1 said:
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No. A diagonal links the corner to one of the other non-adjacent corners. How many of these are there (still with n=10)?
 
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Natasha1 said:
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No. A corner has 2 adjacent corners, and there is no diagonal linking a corner with itself. So, still with n=10, how many diagonals are there starting in one corner?
 
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If you take a triangle with 3 sides (obviously), there are no diagonals, right?
 
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Natasha1 said:
If you take a triangle with 3 sides (obviously), there are no diagonals, right?
Yes, that is correct. A corner of a triangle has two adjacent corners, and that's it. No possibility to draw a diagonal there.
 
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So a 4 sided polygon as only 1 diagonal, am I correct?
 
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Natasha1 said:

The Attempt at a Solution


Can someone just help me to start on this? I know that q = 3 and p = 2 but how?
In future posts, you need to show more of an effort than this.
 
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Natasha1 said:
So a 4 sided polygon as only 1 diagonal, am I correct?
No. A 4 sided polygon has 2 diagonals.
 
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But apart from trial and error is there any other way I can do this?

4 sides polygon
1/2 . 4 . (4-3) = 2 diagonals
5 sides polygon
1/2 . 5 . (5-3) = 5 diagonals
6 sides polygon
1/2 . 6 . (6-3) = 9 diagonals

And so on...
 
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Mark44 said:
In future posts, you need to show more of an effort than this.
Apologies
 
  • #14
Natasha1 said:
But apart from trial and error is there any other way I can do this?
Yes, there is. I was trying to get you there by reasoning.

So, still with n=10, how many diagonals are there starting in one corner?
 
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4 sided polygon
1/2 . 4 . (4-3) = 2 diagonals
5 sided polygon
1/2 . 5 . (5-3) = 5 diagonals
6 sided polygon
1/2 . 6 . (6-3) = 9 diagonals
...
10 sided polygon
1/2 . 10 . (10-3) = 35 diagonals
 
  • #16
Is there anyway I can do this problem without doing it like i have, guessing what p and q are...

How can one prove it?
 
  • #17
Natasha1 said:
Is there anyway I can do this problem without doing it like i have, guessing what p and q are...

How can one prove it?
By reasoning how many diagonals there are. It is possible, it is easy, but you have to try it.

Outline:
Take one corner, and compute the number of diagonals it lies on. We were almost there: there is a diagonal linking that corner to any other corner, except itself and its two adjacent corners. That makes n - 1 - 2 = n -3 diagonals from that one corner.
Now, this is the case for each one of the n corners. So now you can compute the total number of diagonals, but don't forget that each diagonal connects two corners.

Good night.
 
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Got it thanks!
 

1. How do you calculate the number of diagonals in a polygon?

The formula for calculating the number of diagonals in a polygon is n(n-3)/2, where n is the number of sides of the polygon. This formula applies to all regular and irregular polygons.

2. Can the number of diagonals in a polygon be negative?

No, the number of diagonals in a polygon cannot be negative. The formula for calculating the number of diagonals will always result in a positive integer, as the number of sides and the number of diagonals must be positive values.

3. Is there a difference between a diagonal and a side of a polygon?

Yes, there is a difference between a diagonal and a side of a polygon. A diagonal is a line segment that connects two non-adjacent vertices of a polygon, while a side is a line segment that connects two adjacent vertices.

4. Can a polygon have an infinite number of diagonals?

No, a polygon cannot have an infinite number of diagonals. The maximum number of diagonals a polygon can have is equal to the number of sides minus three.

5. How many diagonals does a regular polygon have?

A regular polygon has n-3 diagonals, where n is the number of sides. For example, a regular pentagon (5 sides) has 5-3=2 diagonals, while a regular hexagon (6 sides) has 6-3=3 diagonals.

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