The **Exterior Angle Theorem** is a key concept in geometry. It states that:

**The measure of an exterior angle of a triangle is equal to the sum of the measures of the two non-adjacent interior angles.**

### Example:
If you have a triangle with interior angles \( \angle A \), \( \angle B \), and \( \angle C \), and you extend the side opposite \( \angle C \), creating an exterior angle \( \angle D \), then according to the Exterior Angle Theorem:
\[
\angle D = \angle A + \angle B
\]

### Application Steps:
1. Identify the exterior angle of the triangle.
2. Find the two non-adjacent interior angles.
3. Use the theorem to set up the equation: Exterior angle = Sum of the two non-adjacent interior angles.
4. Solve for the unknown angle if needed.

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### Example Problem:
In a triangle, if one exterior angle is \(120^\circ\) and one of the opposite interior angles is \(45^\circ\), find the other interior angle.
- By the theorem: \( 120^\circ = 45^\circ + \text{other interior angle} \).
- Solving: \(\text{other interior angle} = 120^\circ - 45^\circ = 75^\circ \).

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### Relative Questions:
1. How do you calculate interior angles of a triangle using exterior angles?
2. Can the Exterior Angle Theorem apply to polygons other than triangles?
3.

Understand the Exterior Angle Theorem, which states that the measure of an exterior angle of a triangle is equal to the sum of the two non-adjacent interior angles. Learn how to apply this theorem with an example where one exterior angle is 120° and an opposite interior angle is 45°.

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