[tex]\Delta ABC \: ,AB=5,BC=7,AC=6[/tex]
[tex]cos\widehat{A}=?[/tex]
Ne folosim de teorema cosinusului :
[tex]{BC}^{2}={AB}^{2}+{AC}^{2}-2\times AB \times AC\times cos\widehat{A}[/tex]
[tex] {7}^{2} = {5}^{2} + {6}^{2} - 2 \times 5 \times 6 \times cos\widehat{A}[/tex]
[tex]49 = 25 + 36 - 60cos\widehat{A}[/tex]
[tex]60cos\widehat{A} = 61 - 49[/tex]
[tex]60cos\widehat{A} = 12[/tex]
[tex]cos\widehat{A} = \frac{12}{60} [/tex]
[tex]cos\widehat{A} = \frac{1}{5} [/tex]
