SOLUTION: If I know the polygon to get the number of diagonals I can use the formula n(n-3)/2 where n=the number of sides of the polygon. How can I find the polygon when I know the num

Question 602117: If I know the polygon to get the number of diagonals I can use the formula n(n-3)/2 where n=the number of sides of the polygon. How can I find the polygon when I know the number of diagonals that can be drawn inside? ex. Which polygon has 20 diagonals that can be drawn inside? Is there a formula for this question?
Answer by jim_thompson5910(35256) (Show Source): You can put this solution on YOUR website!
Let d = number of diagonals

The formula to find the number of diagonals 'd' is

Solve this for n to find the number of sides given the number of diagonals

Now use the quadratic formula to solve for 'n'

or

So things are a bit uglier, but now you have a formula for 'n' given the number of diagonals 'd'.
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