Triangles

TRIANGLES
A triangle is a three-sided polygon. In fact, it is the polygon with the least number of sides. 
We write ∆DABC instead of writing Triangle ABC. The three sides of the triangle are AB , BC and CA . The three angles are ∠BAC, ∠BCA and ∠ABC. The points A, B and C are called the     vertices of the triangle.

Being a polygon, a triangle has an exterior and an interior. In the figure (ii), P is in the interior of the triangle, R is in the exterior and Q on the triangle.