5.3.1.13 Construction of secrets

Condition: on the ribs \ (s a \) and \ (s c \) pyramids \ (s a b c \) the points \ (p \) and \ (q \) are set respectively. Build the cross -section of the pyramid with a plane parallel to the straight \ (b p \) and \ (a q \) and passing through the point \ (l \), taken in the face \ (a b c \)