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Find the number of diagonals by joining ...

Find the number of diagonals by joining the vertices of a polygon of n sides.

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No. of vertices of n-sides polygon = n
No. of straight lines by joining two-two vertices `= .^(n)C_(2)`
In these lines, the sides and the diagonals of the polygon are included.
`:.` Required diagonals `= .^(n)C_(2) - n`
`= (1)/(2) n(n-1) -n`
`=(1)/(2)n (n-3)`.
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