Pls answer the following marked questions...

Let ABCD be a cyclic parallelogram.

∠A + ∠C = 180° (Opposite angles of a cyclic quadrilateral) ... (1)

We know that opposite angles of a parallelogram are equal.

∴ ∠A = ∠C and ∠B = ∠D

From equation (1),

∠A + ∠C = 180°

⇒ ∠A + ∠A = 180°

⇒ 2 ∠A = 180°

⇒ ∠A = 90°

Parallelogram ABCD has one of its interior angles as 90°. Therefore, it is a rectangle.
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