Question

Intr-un triunghi ABC avem cos A= 2a/bc daca si numai daca:
1) ab=a^2+b^2
2) 5a^2=b^2+c^2
3) 2c^2=a^2+b^2
4) 3a^2=b^2+c^2

Answer 1

[tex]\it cosA = \dfrac{2a^2}{bc} \ \ \ \ (1) \\ \\ \\ Th.\ cosinusului \Longrightarrow cosA = \dfrac{b^2+c^2-a^2}{2bc} \ \ \ \ (2) \\ \\ \\ (1), (2) \Longrightarrow 5a^2=b^2+c^2[/tex]