5.3.2.23 Various Problems in the Space

Condition: in the correct pyramid \ (s a b c d \) the dihedral angle in the side edge is equal to \ (120^{\ circ} \). On the ribs \ (s c \) and \ (s d \) the points \ (k \) and \ (l \) are taken respectively - the middle of these ribs. Find the angle formed by a straight \ (d k \) with a straight line \ (a c \).