Question 1192572
<pre>
Here's the concave quadrilateral before drawing in BD.
A concave polynomial is on that has at least 1 "sunk-in" place.
The quadrilateral below is "sunk in" at C:

{{{drawing(400,300,-1,11,-1,8,
locate(0,0,A),locate(10,0,B),locate(5.7,2.6,C), locate(7.25,7.3,D),
locate(.6,.85,44^o),
line(0,0,10,0), line(10,0,5.40348282892,2.183148799),
line(5.40348282892,2.183148799,7.193398003,6.94658),
line(7.193398003,6.94658,0,0) )}}}

Here it is after drawing BD, and figuring out all the angles:

{{{drawing(400,300,-1,11,-1,8,

green(line(7.193398003,6.946583705,10,0)),

locate(0,0,A),locate(10,0,B),locate(5,2.4,C), locate(7.25,7.3,D),
locate(.6,.85,44^o),locate(7.8,.85,18^o),locate(8.7,1.45,18^o),
locate(6.8,6.1,18^o),locate(6,6.05,18^o),
locate(5.62,2.9,144^o),locate(3.65,2.52,216^o),
red(arc(5.40348282892,2.183148799,1.3,-1.3,70,330)),
line(0,0,10,0), line(10,0,5.40348282892,2.183148799),
line(5.40348282892,2.183148799,7.193398003,6.94658),
line(7.193398003,6.946583705,0,0) )}}}

All you need to know to figure out those angles:
1. The three angles of a triangle have sum 180<sup>o</sup>.
2. Base angles of an isosceles triangle are equal in measure.
3. When you bisect an angle you divide its measure by 2.

Edwin</pre>