The triangle $ABC$ is isosceles at $A$ with $\angle BAC=20^o$. The points $D$ and $E$ are on the sides $[AB]$ and $[AC]$, respectively. We know that $\angle EBC=40^o$ and $\angle BCD=50^o$. What is the measure of $\angle BED$?
What I did: since $ABC$ is isosceles at $A$ and $\angle BAC=20^o$, we get that $\angle ABC=\angle BCA=80^o$. Therefore, we have $\angle DBE=40^o$ and $\angle BCD=50^o$. Moreover, the lines $(EB)$ and $(CD)$ are perpendicular. It follows that $\angle BEC=60^o$ and $\angle BDC=50^o$.
All the angles of the "butterfly" BDCE are known, and we know the angle $\angle EBC$. So the $\angle BED$ should not be too complicated to get, but I am stuck. Any idea?
Best Answer
As $\angle BCD = 50^0, \angle CBE = 40^0$, we have $\angle BFC = 90^0$.
Please note as $\angle DBF = \angle CBF$ and $BF \perp CD$, $BF$ is perpendicular bisector of $CD$. So, $DF = FC$ and $EF \perp CD$.
That leads to $\triangle DFE \cong \triangle CFE$. So, $\angle CDE = \angle DCE = 30^0$ and $\angle BED = \angle BEC = 60^0$.