Explanation:
the rules of the angles of 2 lines intersecting each other.
that means the angles on one side of the first line sind the intersection point, are the same as on the other side of the line - just left-right interchanged.
and to have a third line, parallel to the first line, where the exact same angles have to be, where the second line intersects the third line (a parallel line is like a mirror image of the first line : every look and feel has to be the same).
and then remember : all angles around a single point in one side of a line must be together 180°.
so, if the support beam has an angle of 125° with the top "line", the second angle there (the exterior Angie at that point) is then 180 - 125 = 55°.
this is all at the bottom side of the top "line".
these angles are therefore left-right exchanged the same on the top side of the top "line". so, 55° and 125° instead of 125° and 55°.
these angles at the top of the top "line" must be now exactly the same at the top of the parallel bottom "line" of the truss.
so, the interior angle there is 55°. and the exterior angle is 125°.