<u>Collinear points</u>: Three points A, B and C are said to be collinear if they lie on the same straight line.
There points A, B and C will be collinear if AB + BC = AC as is clear from the adjoining figure.
In general, three points A, B and C are <u>collinear if the sum of the lengths of any two line segments among AB, BC and CA is equal to the length of the remaining line segment</u>, that is, either AB + BC = AC or AC +CB = AB or BA + AC = BC.
In other words,
<u>There points A, B and C are collinear if:</u>
(i) AB + BC = AC i.e.,
Or, (ii) AB + AC = BC i.e. ,
Or, AC + BC = AB i.e.,