6. The horizontal angles from the boat between A and B and B and C, the stations on the shore are respectively ?1 and ?2. The distances AB = L1 and BC = L2. For calculating the angle ?2 at C between the boat and station B is obtained by using the following formula: where (? is horizontal angle between A and C at B).
10. According to the recommendations of International Navigational Congress in 1912, the ratio of length to width at the entrance for passenger vessels is :