Prove that opposite angles of an isosceles trepezium are equal .

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