急急急!!!!!! 要过程!别跳步. 完后加悬赏!!1.已知:如图,AB是⊙O的直径,弦CD⊥AB于E,∠ACD=30°,AE=2cm.求DB长.2.已知:如图,△ABC内接于圆,AD⊥BC于D,弦BH⊥AC于E,交AD于F. 求证:FE=EH.3.已知:如图,△ABC内接于⊙O,AM平分∠BAC交⊙O于点M,AD⊥BC于D. 求证:∠MAO=∠MAD.