Lesson on Exterior Angle of a Triangle

An auxiliary line is a line that is added to a figure
to aid in a proof.

An auxiliary line

used in the

Triangle Sum

Theorem

All angles that form a straight line add up to 180°

An auxiliary line

used in the

Triangle Sum

Theorem

A corollary is a theorem whose proof follows
directly from another theorem. Here are two
corollaries to the Triangle Sum Theorem.

The interior is the set of all points inside the
figure. The exterior is the set of all points
outside the figure.

Interior

Exterior

An interior angle is formed by two sides of a
triangle. An exterior angle is formed by one
side of the triangle and extension of an adjacent
side.

∠4 is an exterior angle.

∠3 is an interior angle.

Interior

Exterior

Each exterior angle has two remote interior angles. A
remote interior angle is an interior angle that is not
adjacent to the exterior angle.

Exterior

∠4 is an
exterior angle.

The remote
interior angles
of ∠4 are ∠1
and ∠2.