Áp dụng định lí côsin, ta có:
$BC^{2} = AB^{2} + AC^{2} - 2AB. AC. cosA$
$\Leftrightarrow$ $800^{2} = 500^{2} + 700^{2} - 2. 500. 700. cosA$
$\Rightarrow$ cosA = $\frac{1}{7}$ $\Rightarrow$ $\widehat{A} \approx 81^{\circ}47'$
$AC^{2} = AB^{2} + BC^{2} - 2AB. BC. cosB$
$\Leftrightarrow$ $700^{2} = 500^{2} + 800^{2} - 2. 500. 800. cosB$
$\Rightarrow$ cosB = $\frac{1}{2}$ $\Rightarrow$ $\widehat{B} = 60^{\circ}$
$\widehat{C} = 180^{\circ} - \widehat{A} - \widehat{B} \approx 180^{\circ} - 81^{\circ}57' - 60^{\circ} = 38^{\circ}13'$