完成下面推理过程: 如图,已知∠1 =∠2,∠B =∠C,可推得AB∥CD.理由如下: ∵∠1 =∠2(已知), 且∠1 =∠CGD(______________ _________), ∴∠2 =∠CGD(等量代换). ∴CE∥BF(___________________ ________). ∴∠ =∠C(__________________________). 又∵∠B =∠C(已知), ∴∠ =∠B(等量代换). ∴AB∥CD(________________________________). |