A triangle is a three-sided polygon. It is the polygon with the least
number of sides. It is denoted by the symbol ∆.
We see many triangle shaped objects in our daily life.
Example: Draw a rough sketch of a triangle ABC. Mark a point P in
its interior and a point Q in its exterior. Is the point A in its exterior
or in its interior?
Point A is not in the interior or exterior of ∆ ABC as it is a vertex (a
point where two line segments meet).
a) Identify three triangles in the figure.
b) Write the names of seven angles.
c) Write the names of six line segments.
d) Which two triangles have ∠B as common?
a) The three triangles are ∆ABC, ∆ABD and ∆ACD
b) The angles are ∠BAC, ∠BAD, ∠DAC, ∠ABD, ∠ACD, ∠ADB and
c) The line segments are AB, BC, CA, BD, DC and AD.
d) ∆ABC and ∆ABD have ∠B as common.