Because AB is parallel to CD, angle one is equal to angle two (two straight lines are parallel and the internal dislocation angles are equal);

Because angle one equals angle two, angle two equals angle three, and angle one equals angle three (equivalent replacement);

Because angle 1 is the inner angle of triangle ABC, angle ACB is equal to angle 3.

The above is an example, no matter what kind of questions, this step is essential, and the rest is for you to look at the questions and pictures yourself. The basic knowledge is not good, so I can't do it.