This theorem is also known as the high school exterior angle theorem or Euclid's exterior angle theorem. This states that any exterior angle (∠BCD) of a triangle equals the sum of both interior angles (∠A) and (∠B) at the other 2 triangle vertices.
This is very easy to prove.
This states that when 2 parallel lines (the blue lines) are intersected by a
transversal (the red line), then the alternate exterior angles are equal.
That is ∠ A = ∠ D and ∠ B = ∠ C.

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