Interior Angle
An angle on the interior of
a plane figure.
Examples: The angles
labeled 1, 2, 3, 4, and 5 in the pentagon below
are all interior angles. Angles 3, 4, 5, and 6 in the second example
below are all interior angles as well (parallel
lines cut by a transversal).
Note: The sum of the interior angles
of an ngon is given by the
formula (n – 2)·180°.
For a triangle this sum is 180°,
a quadrilateral 360°,
a pentagon 540°, etc.
Parallel lines cut
by a transversal 

