In $\triangle ABC$, $AB = AC$ and $\measuredangle B = 40^\circ$ . $D$ is a point on $AB$ produced such that $AD = BC$. Join $DC$. Find $\angle DCB$ .
What I Tried: Here is a picture :-
For this problem, I think that I have to find the use of the information that $AD = BC$ , but I cannot find how. I tried angle-chasing, looked for similar triangles and cyclic quadrilaterals, but could not find any which can be that useful.
Can anyone help?
Best Answer
Draw $\angle EAD = \angle ADE = 60^0$. So, $\angle CAE = 40^0$.
$\triangle ACE \cong \triangle BAC$ (by side, side, angle).
As $\triangle ACE$ is isosceles with $AC = CE$, and $\triangle ADE$ is equilateral, with common side $AE$, $CD$ is perpendicular to $AE$.
$\angle BCD = 90^0 - \angle CGF = 90^0 - 80^0 = 10^0 \,$ (as $\angle CGA = 100^0$).