If ABCD is an isosceles trapezium ∠C is equal to:
Properties of an isosceles trapezium:
It has a pair of parallel sides.
It has a pair of opposite sides that are congruent.
Both pairs of opposite angles are supplementary, that is they sum to 180°.
Consecutive angles along both bases are congruent.
Diagonals are congruent.
∴ In an isosceles trapezium the base angles are always equal. Hence, it depends how we name an isosceles trapezium.