Because AC 1? Plane ABC 1, OE? Plane ABC 1, so OE∑ plane ABC1;

(2) connect A 1C 1 because AB=a, A 1C 1 = 2A.

So the quadrilateral ACC 1A 1 is a square, so A 1C⊥AC 1,

Because OE sigma AC1.

So A 1C⊥OE,

And because BD⊥AC, BD⊥AA 1, AC ∩ AA1= A.

So BD⊥ aircraft AA 1C, so BD⊥A 1C,

And because OE∩BD=O, A 1C⊥ plane BDE. ..