3. $\triangle A B C$ is an isosceles triangle, with vertex angle $A$ being $20^{\circ}$. On $A B$, take $A D = B C$, and connect $D C$. Find $\angle B D C$.
(Answer: $30^{\circ}$. Hint: Inside $\triangle ABC$, construct an equilateral triangle $BCN$ with $BC$ as a side, and connect $AN$)