⑵ Connect AC, AC=BC, OC=m, ∴AC=BC=8-m,

In rt δ oac, AC 2 = OA 2+OC 2,

(8-m)^2= 16+m^2,m=3，

∴BC=5。

(3)∫d does not coincide with A, ∴∠DOC is an acute angle.

①∠OCD=90，CD=OC，OC=m，CD=- 1/2m+4，

From m=- 1/2m+4, m=8/3.

②∠ODC = 90°，OD=CD，

Then d is on the vertical line of OC, and DE⊥X axis is on e after passing through d,

OE= 1/2OC= 1/2m，DE=- 1/4m+4，

From OC=2DE, m=2(- 1/4m+4), m= 16/3.