CBSE Class 9 Answered

Error in question the opposite angles are  supplementary can't be equal

In an isosceles trapezium, AB is parallel to DC and the alternate interior angles are supplementary. So, the angles ∠A and ∠D are supplementary.which means 
∠A + ∠D = 180

Similarly, ∠B and ∠C are supplementary.
so,  ∠B + ∠C = 180
Since the trapezium is the isosceles trapezium, the base angles are equal. ∠C = ∠D So by substituting the angles we get
∠A + ∠C = 180
∠B + ∠D = 180 

so the opposite angles are also supplementary