Cách làm cho bạn:
Ta có: $a^{2}=b^{2}+c^{2}-2bc\cos A$
<=> $a^{2}=8^{2}+5^{2}-2.8.5\cos 120^{\circ}=129 (cm) $
=> $a=\sqrt{129} (cm) $
Mặt khác: $\frac{a}{\sin A}=\frac{b}{\sin B}$
=> $\sin B=\frac{b\sin A}{a}=\frac{8.\frac{\sqrt{3}}{2}}{11,36}=0,61$
=> $\widehat{B}=37^{\circ}34'$
=> $C=\widehat{C}=180^{\circ}-\widehat{A}-\widehat{B}=180^{\circ}-120^{\circ}-37^{\circ}34'=22^{\circ}26'$
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