题目如图甲,已知在△ABC\triangle ABC△ABC中,∠ACB=90∘\angle ACB=90^{\circ}∠ACB=90∘,AC=BCAC=BCAC=BC,直线MNMNMN经过点CCC,且AD⊥MNAD\bot MNAD⊥MN于DDD,BE⊥MNBE\bot MNBE⊥MN于EEE.(1)(1)(1)证明:AD+BE=DEAD+BE=DEAD+BE=DE.(2)(2)(2)已知条件不变,将直线MNMNMN绕点CCC旋转到图乙的位置时,若DE=3DE=3DE=3,AD=5.5AD=5.5AD=5.5,则BE=______.BE= \_\_\_\_\_\_.BE=______.知识点：对顶角、邻补角、全等三角形的判定与性质章节：未标注
