By the Exterior Angle Theorem, the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles (the angles that do not form a linear pair with the exterior angle).

Consider \(\displaystyle\triangle{A}{B}{C}\) below:

The exterior angle measures \(\displaystyle{m}\angle{A}{C}{D}={110}∘\) and the two remote interior angles have a sum of \(\displaystyle{m}\angle{A}+{m}\angle{B}={60}∘+{50}∘={110}∘\). Therefore \(\displaystyle{m}\angle{A}{C}{D}={m}\angle{A}+{m}\angle{B}\).

Consider \(\displaystyle\triangle{A}{B}{C}\) below:

The exterior angle measures \(\displaystyle{m}\angle{A}{C}{D}={110}∘\) and the two remote interior angles have a sum of \(\displaystyle{m}\angle{A}+{m}\angle{B}={60}∘+{50}∘={110}∘\). Therefore \(\displaystyle{m}\angle{A}{C}{D}={m}\angle{A}+{m}\angle{B}\).