Because the quadrilateral ABCD is a quadrilateral inscribed in a circle.
So ∠ ABC = ∠ CDE
Because ∠FDE =∠ ADB, ∠ ADB =∠ACB
So ∠ FDE = ∠ ACB
Because DE divides ∠CDF equally.
So ∠ FDE = ∠ CDE
So ∠ ABC = ∠ ACB
So ab = AC
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