Explicație pas cu pas:
[tex]\cos(B) = \frac{c}{a} \\ \cos(C) = \frac{b}{a} \\ {b}^{2} + {c}^{2} = {a}^{2} [/tex]
→
[tex]b\cdot \cos(C) + c\cdot \cos(B) = b\cdot \frac{b}{a}+ c\cdot \frac{c}{a} \\ = \frac{{b}^{2} + {c}^{2}}{a} = \frac{{a}^{2} }{a} = a[/tex]
