It is proved that (1) connects OE, and when e is the midpoint of side SC, OE is the midline of △SAC.

So SA∨OE, (3 points)

Because of SA? Plane BDE, OE? Plane BDE,

So the sa Σ plane BDE. (5 points)

(2) Because SB=SD and O is the midpoint of BD,

So BD⊥SO, (7 points)

Because the quadrilateral ABCD is a square, BD⊥AC, (9 points)

Because AC∩SO=O, BD⊥ plane SAC. ( 1 1)

Is it BD again? Plane BDE,

So the aircraft BDE⊥ aircraft bag. (12)