Show that the points (3, 3), (9, 0) and (12, 21) are the vertices of a right angled triangle.

Let the coordinates of A, B and C be (3, 3), (9, 9) and (12, 21)

Since BC² = AB² + AC²

i.e. 450 = 405 + 45 = 450

Hence by converse of Pythagoras theorem

ABC is a right angled triangle right angled at A i.e. (3, 3)