5.3.1.2 Construction of secrets

The base of the pyramid \ (s a b c d \) is a rhombus with diagonals \ (a c = a, b d = b \). The lateral rib \ (s a \) perpendicular to the plane of the base and equal to C. Build the pyramid section with a plane passing through the point \ (a \) and the middle \ (k \) ribs \ (s c \) parallel to the straight line \ (b d \), and find the area of this section.