An exterior angle of a triangle equals minus the adjacent interior angle.

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An exterior angle of a triangle equals 180° minus the adjacent interior angle.

Step-by-step explanation:

The two angles are said to be adjacent if they have a common vertex and a common side between them.

So, if an angle is exterior to any angle of a triangle, But that particular angle of a triangle is adjacent to the exterior angle. Then, The both angles must lie on the straight line and thus both will form a linear pair.

The measure of the linear pair is always supplementary.

⇒ Exterior angle + Adjacent interior angle = 180°

⇒ Exterior angle = 180° - Adjacent interior angle

Hence, An exterior angle of a triangle equals 180° minus the adjacent interior angle.

i think it would be 42cm