(1) The AB side keeps tangent to the arc EF during rotation, because if you regard AB as a lever, CE is the arm of AB (CE is perpendicular to AB), or in other words, AB keeps the distance from C unchanged.

(2) From the hypotenuse midline theorem, CE=AB/2. From Pythagorean theorem, from Pythagorean theorem, the square of AB plus the square of AD = the square of BD, and the square of AB = the square of BD, so twice the square of AB is equal to the square of BD, and then both sides open the root sign at the same time, that is, the root sign 2 times AB =BD.

Therefore, the CE length can be found. I hope I can help you.

(3) It doesn't matter if you haven't learned radical symbols. In short, the root sign is the inverse of the square, that is, if the square of a =B and A >；; 0, then the root number B = A and the square of the root number 2 is equal to 2.

By the way, the waist is opposite to the ass. If a triangle has one waist, it must have another waist, and the remaining side is the bottom. This is a relative concept, depending on how you look at it. Generally, people call the two adjacent sides of the vertex angle of a triangle two waists.

By the way, Pythagorean Theorem: In a right triangle, the sum of squares of two right-angled sides is equal to the square of the length of the third side (hypotenuse).

Good luck.