Alternate Interior Angles

Definition of Alternate Interior Angles

When two lines are cut by a transversal, the pair of angles formed interior of the two lines and on the opposite of the transversal is called Alternate Interior Angles.

More About Alternate Interior Angles:

If two parallel lines are cut by a transversal, the alternate interior angles are congruent

Examples of Alternate Interior Angles

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

example of Alternate Interior Angles

Video Examples: Alternate Interior Angles

Solved Example on Alternate Interior Angles

Ques: Find the measure of angle m in the figure shown.

example of Alternate Interior Angles

  • A. 120o
  • B. 180o
  • C. 60o
  • D. Insufficient information

Correct Answer: C


  • Step 1: 120o + n = 180o [Straight angle.]
  • Step 2: n = 60o [Solve for n.]
  • Step 3: n = 60o implies m = 60o, because m and n are alternate interior angles and so they are congruent.

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