As shown in the figure, ab ∥ CD, ∠ ACB = 90 °, e is the midpoint of AB, CE = CD, de and AC intersect at point F

As shown in the figure, ab ∥ CD, ∠ ACB = 90 °, e is the midpoint of AB, CE = CD, de and AC intersect at point F

It is proved that: (1) in right triangle ACB, ∵ CE is the middle line of hypotenuse AB, ∵ CE = AE = be = CD, and ∵ ab ∥ CD, ∵ BCDE is parallelogram, ∵ BC ∥ De, ≁ AC ⊥ BC, ≁ de ⊥ AC. (2)