TITLE: Finding angle in a triangle with one median and an isosceles triangle in it
QUESTION [6 upvotes]: In the following figure $AF=BD=DC$ and $AE=EF$. Find the angle $\alpha$.


I tried so many different things (constructing triangles, drawing parallel lines, using Ceva theorem in triangles $ABD$ and $CBE$, ...). Any help for solving this will be much appreciated.
A geomtrical solution (without trigonometry) would be very nice.

REPLY [4 votes]: Here is a purely geometric proof without any trigonometry.

Draw the three lines $a, \, b, \, c$ such that:
$$A \, \in \, a \,\,\text{ and } \,\, a \, || \, BC$$
$$B \, \in \, b \,\,\text{ and } \,\, b \, || \, CA$$
$$C \, \in \, c \,\,\text{ and } \,\, c \, || \, AB$$
Furthermore, let
$$B^* \, = \, a \, \cap \, c \,\,\, \text{ and } \,\,\, A^* = b \, \cap \, c$$
Then, $ABA^*C$ and $ABCB^*$  are parallelograms and furthermore
$$AB = CA^* = CB^*$$ as well as
$$AB^* = BC$$
Apply Menelaus' theorem to the triangle $BCE$ intersected by the line that passes through the collinear points $A, \, F,\, D$:
$$\frac{AB}{AE} \cdot \frac{EF}{CF} \cdot \frac{DC}{BD} = 1$$
Since by assumption $AE = EF$ and $BD = CD$, one concludes that
$$AB = CF$$
Combining this latter fact with the earlier conclusions, one observes that
$$CF = AB = CA^* = CB^*$$ which is possible if and only if triangle $\Delta \, A^*B^*F$ has $90^{\circ}$ angle at vertex $F$, i.e. $\angle \, A^*FB^* = 90^{\circ}$. But that means that:
$$\angle \, AFB^* = 180^{\circ} - \angle \, A^*FB^* = 180^{\circ} - 90^{\circ} = 90^{\circ}$$
Recall that by assumption $AF = BD = DC = \frac{1}{2}\,BC$. But by construction, $ABCB^*$ is a parallelogram and $AB^* = BC$ so
$$\Delta \, AFB^*  \, \text{ is a triangle with the properties that } \, \angle \, AFB^* = 90^{\circ} \,\, \text{ and } \,\, AF = \frac{1}{2}\, AB^* $$
which is possible if and only if $$\angle \, FAB^* = 60^{\circ}$$
However, $AB^*$ is parallel to $BC$ and consequently, by the properties of a pair of parallel lines intersected by a third line,
$$\angle \, ADB = \angle \, DAB^* = \angle \, FAB^* = 60^{\circ}$$