prove that quadrangle is isosceles trapezoid How to prove that quadrangle $ABCD$ is a isosceles trapezoid? 
where $AB$ is parallel to $CD$


There are two given facts:inscribed quadrilateral and AB is parallel to DC
From 1st given fact,
We can know that the sum of opposite angles of inscribed quadrilateral is 180.
angle DAB +angle BCD = 180
From 2nd,
Since AB is parallel to DC,
angle ABC + angle BCD = 180
So, we get angle DAB + angle BCD = angle ABC + angle BCD
        Therefore,                angle DAB = angle ABC
