Question 74
One of the diagonals of an inscribable quadrilateral is a diameter of its circumscribed circle. 
Check that the intersections $E$ and $F$ of the perpendicular lines by $A$ and $C$, respectively, at the straight $BD$ satisfies $$
DE = BF
$$
Check that the intersections $E$ and $F$ of the perpendicular lines by $A$ and $C$, respectively, at the straight $BD$ satisfies $$DE = BF
$$