The main idea behind the Angle Addition Postulate is that if you place two angles side by side, then the measure of the resulting angle will be equal to the sum of the two original angle measures. For this postulate to apply, the vertices, which are the corner points of the angle, have to also be placed together.
The angle in question to be proven is
Therefore,
Hence,
The final answer is m∠ADC= m∠ADB + m∠BDC