1. The problem statement, all variables and given/known data I am reading a book by Chandrasekhar, "Newton's Principia for a Common Reader." I don't understand some notation. 2. Relevant equations (2) A(1)S(1) x A(1)D(1)=(A(1)B(1))^2 What does x means here? A cross product? Could you give me a hint where to find a good introduction to these geometrical operations? Obviously, there is a gap in my background.