1, substitute x=OA= 10 into y=4x/5 to get y=8, so B (10/0,8) and C (0 0,8).

2. Quadrilateral CDEB is not necessarily a diamond.

If the quadrilateral CDEB is a diamond, then CD=CB=OA= 10, OD=6, that is, D (6 6,0) is obtained from Rt△COD.

Substituting b (10/0,8) and d (6 6,0) into k=2 of y=kx+b, and b=- 12, the straight line BD is y=2x- 12.