Cos and Sin in a triangle that has no right angle

Hey MHF!

Here something I found.

I'm still learning geometry again, so this may seem easy for you guys.

The sides of $\displaystyle \Delta ABC$ are $\displaystyle AB = 6$ $\displaystyle BC=4$ and $\displaystyle CA=5$

$\displaystyle \cos <A =$

(The < means the angle)

But!

AB is not the Hypotenuse, since $\displaystyle 16+25=41$

So, I figure this is not a Triangle Rectangle, but something else.

No right angle on this one, right?

I'm still kind of amateur in Geometry, so go easy! hahaha

Thanks (Bow)