alternate interior angles

Definition of Alternate Interior Angles

  • Alternate interior angles are two congruent interior angles that lie on different parallel lines and on opposite sides of a transversal.

Examples of Alternate Interior Angles

  • In the figure shown, l is the transversal  that cut the parallel lines. Angles a and d and angles b and c are alternate interior angles.


Solved Example on Alternate Interior Angles

Find the measure of angle m in the figure shown.


A. 120°

B. 180°

C. 60°

D. Insufficient information

Correct Answer: C


Step 1: 120° + n = 180°              [Straight angle.]

Step 2: n = 60°                            [Solve for n.]

Step 3: n = 60° implies m = 60°, because m and n are alternate interior angles and so they are congruent.

Related Terms for Alternate Interior Angles

  • Interior
  • Congruent
  • Transversal
  • Parallel Line