how to do?

圖片參考：http://xde.xanga.com/d23c5b2b70232171682818/m130353991.jpg

Let ∠ACB=x
∠ABC=∠ACB=x ( base ∠s, isos. △)
∠CDA+∠ABC=90o (ext. ∠ of △)
∠CDA=90o-∠ABC=90o-x
∠ACD+∠ACB+90o=180o (adj. ∠s on st. line)
∠ACD=90o-∠ACB=90o-x=∠CDA
∴AC=AD=AB(sides opp. equal ∠s)
Therefore,AD=AB