The cosine of 30 degrees is 0.86. It is also expressed as the square root of three divided by two. The cosine of an angle is calculated by dividing the length of the side of a right triangle adjacent to the acute angle by the length of the hypotenuse.
The cosine is a trigonometric function. Other functions used in trigonometry are tangent and cosine, and all of these functions are related to one another. The tangent is calculated as the sine divided by the cosine. The cosine is the reverse of the sine and is equal to the quotient of the length of the side of a right triangle opposite the acute angle divided by the length of the hypotenuse.