One way to think of this is to join all the vertices of the polygon to a single point call this point X. For a n-sided poygon, you now have n triangles. The sum of all the angles of each triangle is 180 degrees. So all the angles of all the n triangles add to n*180 degrees.
But, this sum includes all the angles around the point X and these add to 360 = 2*180 degrees. These need to be taken away. So (n-2)*180 degrees.
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Formula: 0.5*(n2-3n) where n is the number of sides
The sum of the first 10 positive integers, using the formula N1 + (N1 + 1) + ... + N2 = N2 * (N2 + 1) / 2 - (N1 - 1) * N1 / 2 is: 55
The formula can be written, with "n" representing a number or value, as n2 or n X n.
tn = n2
1/2*(n2-3n) = number of diagonals Rearranging the formula: n2-3n-(2*diagonals) = 0 Solve as a quadratic equation and taking the positive value of n as the number of sides.