1. As shown in the figure, let a straight line EF, BE⊥EF in E, DF⊥EF in F, diagonal AC, BD intersection O? .

(1) verification: BE+EF=EF? (2)? It is proved that △BDF is an isosceles triangle

2. In RT△ABC, ∠ C = 90, AC=BD, PE⊥AC, PF⊥BC, verification: DE=DF (draw by yourself).

(applied knowledge: 1. The median line on the hypotenuse of an isosceles right triangle is equal to half of the hypotenuse.

2. congruence? 3. The opposite sides of the rectangle are equal. 4. The properties of right isosceles triangle. 5. The practice of auxiliary lines, etc. )?

3. In △ABC, O is the center of gravity of △ABC. (Draw by yourself) (Applied knowledge: 1. What are the properties of midpoint and midline? 2. congruence? 3. What is the area of this triangle? 4. What is the bisector? 5. The nature of the center of gravity)

(1) verification: OD = 1/2ao.

(2) If S△BOD=3, find S△ABC.