Determine whether it is possible to find a circumscribable, isosceles "true" trapezoid (in which the parallel sides are not congruent) that has each of the following properties. Justify the answer.
a) A diagonal bisects an angle of the trapezoid.
b) The trapezoid is cyclic
c) The diagonals are perpendicular to each other
Give examples of circumscriptible and inscriptible trapezoids.