So the definition of the cosine of an angle of a right triangle is [tex] \frac{adjacent}{hypotenuse} [/tex]. You may remember this by SohCahToa (sine is opposite over hypotenuse, cosine is adjacent over hypotenuse, tangent is opposite over adjacent).
Remember, adjacent refers to the side next to the angle in question, and the side can't also be the hypotenuse. The hypotenuse is that longest side and is opposite the right angle.
We have the cosine of 30°. We write
[tex] \cos(30)= \frac{adj}{hyp} = \frac{ \sqrt{3}}{2} [/tex]
