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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