Daca intr-un triunghi se dau lungimile a 2 laturi si unghiul dintre ele si se cere a 3-a latura folosim:
Teorema cosinusului numita si teorema lui Pitagora generalizata.
[tex]\displaystyle\bf\\BC^2=AB^2+AC^2-2\cdot AB\cdot AC\cdot\cos(\widehat{BAC})\\\\BC^2=6^2+10^2-2\cdot 6\cdot 10\cdot\cos(60^o)\\\\BC^2=6^2+10^2-2\cdot 6\cdot 10\cdot\frac{1}{2}\\\\BC^2=36+100-6\cdot 10\\\\BC^2=36+100-60\\\\BC^2=76\\\\BC=\sqrt{76}=\sqrt{4\cdot19} \\\\\boxed{\bf~BC=2\sqrt{19}}[/tex]