How many chords can be drawn through $21$ points on a circle?

There are $n$ points in a plane,of which $p$ points are collinear. How many lines can be formed from these points?

There are $10$ points on the line segment $AB$ excluding $A$ and $B$,and $8$ points on the line segment $AC$ excluding $A$ and $C$. The number of triangles formed using these $18$ points (excluding $A, B,$ and $C$) is:

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

Some identical balls are arranged in rows to form an equilateral triangle. The first row consists of one ball,the second row consists of two balls and so on. If $99$ more identical balls are added to the total number of balls used in forming the equilateral triangle,then all these balls can be arranged in a square whose each side contains exactly $2$ balls less than the number of balls each side of the triangle contains. Then the number of balls used to form the equilateral triangle is

There are $16$ points in a plane out of which $6$ are collinear. How many lines can be drawn by joining these points?