Hello,
Here I am providing you with link for complete proof for the problem you asked
https://www.google.co.in/url?sa=t&source=web&rct=j&url=https://www.meritnation.com/ask-answer/question/prove-that-the-area-of-a-right-angled-triangle-of-given-hypo/application-of-derivatives/2885836&ved=2ahUKEwjq74Df4NnfAhWVYisKHZI0DQ0QFjAIegQIBRAB&usg=AOvVaw1QFP0tufOgu1ipxMYH0fsP&cshid=1546798454523
Hope this helps ...
Question : In $\triangle {ABC}$, D is a point on BC such that $\angle {ADB}=2 \angle {DAC}, \angle {BAC}=70^{\circ}$ and $\angle {B}=56^{\circ}$. What is the measure of $\angle A D C$?
Option 1: $72^{\circ}$
Option 2: $54^{\circ}$
Option 3: $74^{\circ}$
Option 4: $81^{\circ}$
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