b) cos ˆ N M P = M N^ 2 + M P^ 2 − N P ^2 /2 M N . M P .

b) \(\cos \widehat {{\rm{NMP}}} = \frac{{{\rm{M}}{{\rm{N}}^2} + {\rm{M}}{{\rm{P}}^2} - {\rm{N}}{{\rm{P}}^2}}}{{2{\rm{MN}}.{\rm{MP}}}}.\)