SOLUTION
Consider the equation:
[tex]\angle A+\angle B+\angle C=\angle C+\angle D[/tex]
Substracting angle C from both sides gives:
[tex]\begin{gathered} \angle A+\angle B+\angle C-\angle C=\angle C-\angle C+\angle D \\ \angle A+\angle B=\angle D \end{gathered}[/tex]
Therefore the resulting equation is:
[tex]\angle A+\angle B=\angle D[/tex]
This implies that the exterior angle is the sum of the two opposite angles.
