DẠNG 1. HỆ THỨC LƯỢNG TRONG TAM GIÁC

\({{\rm{a}}^2} = {{\rm{b}}^2} + {{\rm{c}}^2} + 2{\rm{bc}} \cdot \sin {\rm{A}}.\)

\({a^2} = {b^2} + {c^2} - 2bc \cdot \sin A.\)

\({a^2} = {b^2} + {c^2} + 2bc \cdot \cos A.\)

\({a^2} = {b^2} + {c^2} - 2bc \cdot \cos A.\)

\({\rm{BC}} = 2{\rm{R}}\cos {\rm{A}}.\)

\(BC = 2R\sin A.\)

\({\rm{BC}} = {\rm{R}}\sin {\rm{A}}.\)

\({\rm{BC}} = {\rm{R}}\cos {\rm{A}}.\)

\(\frac{1}{2}{\rm{bc}} \cdot \sin {\rm{A}}.\)

bc. \(\sin {\rm{A}}.\)

\(\frac{1}{2}{\rm{bc}} \cdot \cos {\rm{A}}.\)

bc. \(\cos {\rm{A}}.\)

\(\sin (a + b) = \sin a\cos b - \cos a\sin b.\)

\(\sin (a + b) = \cos a\cos b + \sin a\sin b.\)

\(\sin (a + b) = \cos a\cos b - \sin a\sin b.\)

\(\sin (a + b) = \sin a\cos b + \cos a\sin b.\)

\(\cos (a + b) = \sin a\cos b - \cos a\sin b.\)

\(\cos (a + b) = \cos a\cos b + \sin a\sin b.\)

\(\cos (a + b) = \cos a\cos b - \sin a\sin b.\)

\(\cos (a + b) = \sin a\cos b + \cos a\sin b.\)