SOLUTION: What is an algebraic expression for the number of diagonals in any polygon of n sides?

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Question 266167: What is an algebraic expression for the number of diagonals in any polygon of n sides?
Answer by palanisamy(496) About Me  (Show Source):
You can put this solution on YOUR website!
A polygon has n sides and n vertices.
A diagonal is obtained by joining two vertices.
Total number = nC2 = n*(n-1)/2
But this includes the n sides also.
So the number of diagonals = n*(n-1)/2 -n
=[n*(n-1)-2n]/2
= n*(n-1-2)
= n(n-3)/2