Dear user,
The angle BAB' would be equal to DCD'.
To understand it better, take the example of a door which has a slant cutting at the top(not parallel to the base). Let the upper line is AB and the baseline is CD now, the axis of door(from where it is hinged to the wall) is perpendicular to CD i.e. the base of the door.
Now, if you open(rotate) the door from its axis, it goes from plane ABCD to AB'CD' and it is quite obvious that the rotation has been done along with AC and both the angles BAB' and DCD' remains the same(the door rotates a uniform angle irrespective of bottom, top or middle).
Hope I was able to help.
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