[tex]cos \: k = \frac{adj}{hyp} = \frac{5}{13} [/tex]
//
I'm unsure about what to do after that but you can:
[tex]cos( \frac{5}{13} ) = 0.999977469[/tex]
[tex]cos \: k = 1.00[/tex]
or
[tex] \frac{5}{13} = 0.384615385[/tex]
[tex] = 0.38[/tex]
I think the first one's safer.
