In the plane BCF, BM is perpendicular to CF, the vertical foot is M, including the bottom CDEF, and E is perpendicular to EN and CF,

Vertical foot is n, horizontal m is MT, vertical EG, horizontal EG extension line is t, connecting BT,

It is easy to prove that BM is perpendicular to the bottom surface, so the angle BTM is the plane angle of the dihedral angle.