where '':<math>\cos{\frac{\phi}{2}}=\sqrt{\frac{(s'' is the [[semiperimeter]].-b)(s-d)(b+d)^2}{(ab+cd)(ad+bc)}}</math><ref name=Durell>{{citation|last1=Durell|first1=C. V.|last2=Robson|first2=A.|title=Advanced Trigonometry|publisher=Dover|year=2003}}.</ref>{{rp|p.31}}