Answer is in the attachment
Step-by-step explanation
*Image is not drawn to scale
A triangle sum theory says the sum of the angles of a triangle is 180. I gave each interior angle a degree of 60.
∠ 1 + ∠ 2 + ∠ 3 = 180
60 + 60 + 60 = 180.
Now the “exterior angle theorem” says the exterior angle will equal the sum of the two opposite interior angles.
∠1 + ∠2 = ∠4
60 + 60 = 120 degrees exterior angle
Now to prove this, we use the “straight angle theorem” that says a straight angle = 180.
∠3 + ∠4 = 180
60 + 120 = 180
This is true and proves our exterior angle theorem.