⑴.作OE⊥AB.E∈AB..设AB＝4a,则OE=2a.ED=a.OE²+ED²＝OD².即4a²+a²＝cos²α+sin²α＝1.5a²＝1.a＝√5/5.∴AB＝4a＝4√5／5.⑵.作DF⊥OA.F∈OA.AD=a.=DA=√2a／2.而OA＝2√2aOF＝（2√2-√2/2）a＝3√2a／2od的斜率＝DF／OF＝（√2a／2）／（3√2a／2）＝1/3.
