* deliab

∴∠B=∠CED

∠∠B =∠C

∴∠C=∠CED

△ CDE is an isosceles triangle.

∴CD=DE

AB = CD

∴AB=DE

∴ABED is a parallelogram

∴∠a=∠bed=∠c+∠cde=∠cde+∠ced=∠cde+∠ade=∠ade

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