If you want to remember the trig formulas, I suggest the “SOHCAHTOA” or the “Some old hippie caught another hippie tripping on acid method.” It combines all three principle formulas together. So, SOH means sine equals opposite over hypotenuse, and CAH means cosine equals adjacent over hypotenuse, and TOA means tangent equals opposite over adjacent.
Cosine equals adjacent/hypotenuse.