The equation is solved using

You can choose r from n possibilities in possible ways.

You can line up r from n possibilities in different ways

Binomial Theorem :

For any triangle ∆ABC ,

The straight line passing through has gradient and equation

The distance between two points, is

A straight linepassing though points A and B is given by

The angle between two vectors is given by The angle between two lines and is given by

To change to a straight line equation, take logs

and