Answer:
I just answered this on your other thread, but just in case you haven't seen it, here is a copy of my answer.
Explanation:
The arrows on line AB and line CD mean that those lines are parallel.
So, ∠a and ∠e are a pair of "corresponding" angles and are therefore equal. So, ∠a = ∠e
As line AB and CD are parallel, ∠b and ∠d are a pair of "alternate angles" (z-angle). Therefore, ∠b = ∠d
If ∠a = ∠e and ∠b = ∠d then we can say that ∠a + ∠b = ∠e + ∠d
The angles on a straight line (AE) add up to 180°.
Therefore ∠c + ∠d + ∠e = 180°
If we substitute ∠a + ∠b = ∠e + ∠d into ∠c + ∠d + ∠e = 180°
we can say that ∠c + ∠b + ∠a = 180°
Thus proving that the sum of the interior angles of a triangle is 180°