ANSWER and EXPLANATION
We have the triangle ABC with its base extended from C to D.
The external angle in the diagram is
The external angle theorem states that the external angle of a triangle is equal to the sum of the two non-adjacent angles.
In other words:
The angles in a triangle sum up to 180 degrees.
This means that:
Angle Angles on a straight line sum up to 180 degrees.
This means that:
From (1), make
From (2), make
Equate (3) and (4):
180 -
Make
=>
It is proven.
