Before you study this topic, you are adviced to read:
Interior Angles of a Triangle
Exterior Angles of a Triangle
In the following diagram you can see exterior angles and interior angles.
Let’s find interior opposite angles in the following ways:
∠ 3 & ∠ 5 are Interior Opposite Angles to exterior ∠ 1, as shown in the following diagram:
∠ 5 & ∠ 4 are Interior Opposite Angles to exterior ∠ 2, as shown in the following diagram:
∠ 3 & ∠ 4 are Interior Opposite Angles to exterior ∠ 6, as shown in the following diagram:

