The number of triangles formed by the diagonals of a 21 sided polygon is C(21,3) = 21x20x19/3x2x1 = 1,330.
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98
18 triangles
There are 90 diagonals in a 15-sided polygon.
A 4-sided polygon has 2 diagonals. A 5-sided polygon, 5. 6-sided, 9. By induction, an n-sided polygon has 'n*(n-3)/2' diagonals. "n" is how many sides a shape has. so, (7)*(7-3)/2 now we have, 7*4/2 so, 28/2 = 14 Therefore, a 7-sided polygon would have 14 diagonals.
The number of triangles that can be formed within a regular polygon depends on the number of sides the polygon has. For an n-sided polygon, where n is greater than or equal to 3, you can form n-2 triangles within the polygon. This is because each triangle is formed by connecting one vertex to any other two non-adjacent vertices. So, for example, in a regular pentagon (5-sided polygon), you can form 5-2 = 3 triangles.