C-03-Q066mediumsingle_mcqPtolemy's theorem for a cyclic quadrilateral ABCDABCDABCD with diagonals ACACAC, BDBDBD states —aAC⋅BD=AB⋅CD+BC⋅ADAC \cdot BD = AB \cdot CD + BC \cdot ADAC⋅BD=AB⋅CD+BC⋅ADbAC⋅BD=AB⋅BC+CD⋅ADAC \cdot BD = AB \cdot BC + CD \cdot ADAC⋅BD=AB⋅BC+CD⋅ADcAC+BD=AB+BC+CD+ADAC + BD = AB + BC + CD + ADAC+BD=AB+BC+CD+ADdAC⋅BD=AB⋅AD−BC⋅CDAC \cdot BD = AB \cdot AD - BC \cdot CDAC⋅BD=AB⋅AD−BC⋅CD