What is the number of diagonals in an octagon?

Let $P_1, P_2, \ldots, P_{15}$ be $15$ points on a circle. The number of distinct triangles formed by points $P_i, P_j, P_k$ such that $i+j+k \neq 15$ is

$A$ regular polygon has $20$ sides. The number of triangles that can be drawn by using the vertices but not using the sides are

The number of rectangles that can be obtained by joining four of the twelve vertices of a $12$-sided regular polygon is

How many different (mutually non-congruent) trapeziums can be constructed using four distinct side lengths from the set $\{1, 2, 3, 4, 5, 6\}$?

Out of $n$ points in a plane,$p$ points are collinear. (No three of the remaining points are collinear). The number of lines that can be drawn passing through these points is: