Prove that a cyclic parallelogram is a rectangle.

Step-by-step explanation:

let ABCD be cyclic parallelogram.

Then ∠A=∠C

as it is cyclic so ∠A+∠C=180

∠C+∠C=180

2∠ C=180

∠C=90

So ∠A=90

similarly ∠B=∠D=90

so it is a rectangle.