5) [tex]\overline{AB}=(x_B-x_A)\overline{i}+(y_B-y_A)\overline{j}=(-1-2)\overline{i}+(3+1)\overline{j}=-3\overline{i}+4\overline{j}\Rightarrow a=-3,\,b=4[/tex]
6) Aplicăm teorema cosinusului:
[tex]\cos B\displaystyle=\frac{AB^2+BC^2-AC^2}{2\cdot AB\cdot BC}=\frac{16+3-7}{2\cdot8\cdot\sqrt3}=\frac{12}{8\sqrt3}=\frac{3}{2\sqrt3}=\frac{3\sqrt3}{2\cdot 3}=\frac{\sqrt3}{2}\\\\\Rightarrow B=30^\circ[/tex]
