First find the number of sides of the polygon and let it be x:
Rearrange the diagonal formula: 0.5*(x2-3x) = 135 into a quadratic equation
Which is: x2-3x-270 = 0 and solving the equation gives x positive value of 18
Therefore the polygon has 18 sides
Sum of interior angles: (18-2)*180 = 2880 degrees
Each interior angle: 2880/18 = 160 degrees
If it is a regular polygon with each interior angle of 120 degrees then it is an hexagon which will have 0.5((6^2)-3*6) equals 9 diagonals
Only if the polygon is "regular".
Only when it is a regular polygon that all interior angles are of equal measure
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Only when the polygon is a regular convex polygon. Such as an equilateral triangle, or a square, or a regular pentagon.
If it is a regular polygon with each interior angle of 120 degrees then it is an hexagon which will have 0.5((6^2)-3*6) equals 9 diagonals
A regular polygon having an interior angle measure of 168o has 30 sides and is called a triacontagon.
Only if the polygon is "regular".
Only when it is a regular polygon that all interior angles are of equal measure
If the measure of an interior angle of a regular polygon is 140o, the polygon is a nonagon (sometimes called an ennagon) having 9 sides.
A pentagon (five sided regular polygon) has interior angles of 108°.
The measure of each interior angle of a fifteen sided regular polygon, a pendedecagon, is 156o.
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A 13 sided regular polygon
The interior angle of an octdecagon, an 18 sided regular polygon, is 160o.
A regular polygon with 4 sides is a square. The measure of any interior angle of a square is 90 degrees.
A regular octadecagon (an eighteen sided polygon) would have nine diagonals.