5.3.2.21 Various problems in the space

condition: At the base of the pyramid lies a square \( A B C D \), and its side edge \( S B \) is perpendicular to the plane of the base and \( S B=A B . \quad \) On the edge \( S C \) a point \( M \) is taken - the middle of this edge. Find the dihedral angle \( S A B M \).