We have a triangle $ABC$ with $AB = 6.5$ , $BC=3.5$ and $AC = \sqrt{61}$. Can we calculate $\angle A$ using the law of cosines? I tried it but I kept getting wrong answers, so I'm wondering if it's possible?
I tried:
$c^2=a^2+b^2-2ab\cos\gamma$
$(3.5)^2= (6.5)^2 + 61 -6.5\sqrt{61}\cos\gamma$
$91 = 6.5\sqrt{61}\cos\gamma$
$\cos\gamma = 1...$ which is impossible