SSC CGL 26 Oct 2014 Shift 1 Paper

Sum of opposite angles of a concyclic quadrilateral = 180°
∴ ∠ A + ∠ C = 180°
In parallelogram ABCD,
∠ A + ∠ B = 180°
∴ ∠ A + ∠ C = ∠ B + ∠ A
⇒ ∠ B = ∠ C
Again, ∠ B + ∠ D = 180°
∠ A + ∠ D = 180° ⇒ ∠ A = ∠ B
∴ ∠ A = ∠ B = ∠ C = ∠ D = 90°