Note that if a cyclic quadrilateral is a trapzeium , then it is an isosceles trapzeium . If a pair of the opposite sides are equal in length , then it is also an isosceles trapzeium .
To prove the first line , Let is a cyclic quadrilateral , assume it is a trapzeium , say , then we have so thus it is an isos. trapzeium .
I give the proof of the second line to you .
By applying the above property , we find that so