Proof method: connecting CM, according to the fact that the middle line on the hypotenuse of a right triangle is equal to half of the hypotenuse, we can get.

Cm = half of ab. That is CM=CD and △MCD is an isosceles triangle.

At the same time, CM=MB, angle B= angle MCB=2, and angle d (the outer angle of a triangle is equal to the sum of two non-adjacent inner angles) are obtained.