Ta có: a2=b2+c2−2bccosA
<=> a2=82+52−2.8.5cos120∘=129(cm)
=> a=129(cm)
Mặt khác: asinA=bsinB
=> sinB=bsinAa=8.3211,36=0,61
=> B^=37∘34′
=> C=C^=180∘−A^−B^=180∘−120∘−37∘34′=22∘26′