Question 118490
If you draw the diagram, you will see that these two perpendicular lines form right triangles AED and BFC inside the parallelogram. You can prove that these 2 triangles are congruent by the AAS theorem:
- Angle BFC is congruent to Angle AED since they are both right angles (given)
- Angle EAC is congruent to Angle BCF since opposite angles of a parallelogram are congruent.
- Line BC is congruent to Line AD since opposite sides of a parallelogram are congruent.
So you have now shown that the 2 triangles are congruent by AAS.
Since the two triangles are congruent, then by definition of congruent triangles, BF is congruent to DE. 
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