Ta có: $BC^{2} = AC^{2} + AB^{2} - 2.AB.AC.\cos \widehat{A}$
<=> $BC= m^{2} + n^{2} - 2.m.n.\cos 120^{\circ}$
<=> $BC = m^{2} + n^{2} + mn$
<=> $BC=\sqrt{m^{2}+n^{2}+m.n}$
Ta có: $BC^{2} = AC^{2} + AB^{2} - 2.AB.AC.\cos \widehat{A}$
<=> $BC= m^{2} + n^{2} - 2.m.n.\cos 120^{\circ}$
<=> $BC = m^{2} + n^{2} + mn$
<=> $BC=\sqrt{m^{2}+n^{2}+m.n}$