Similarty Condition of two polygon

In a convex polygon of 6 sides two diagonals are selected at random. The probability that they interesect at an interior point of the polygon is

A polygon has prime number of sides. Its number of sides is equal to the sum of the two least consecutive primes. The number of diagonals of the polygon is

If a polygon has 8 sides, then the number of diagonals of the polygon is _____.

If a polygon has 6 sides, then the number of diagonals of the polygon is _______

A regular polygon has 20 sides. Find the number of diagonals of the polygon.

The interior angles of a regular polygon measure 150^(@) each.The number of diagonals of the polygon is

The interior angles of a regular polygon measure 120^(@) each.The number of diagonal of the polygon

If each interior angle of a regular polygon is 135^@ , then the number of diagonals of the polygon is equal to