5.3.2.21 Various Problems in the Space

Condition: at the base of the pyramid lies the square \ (a b c d \), a e lateral rib \ (s b \) perpendicular to the plane of the base and \ (s b = a b. \ quad \) on the rib \ (s c \) point \ (m \) - the middle of this rib. Find the dual -sided angle \ (s a b m \).