To find the value of cos 30, we need to use the trigonometric definition of cosine.
The cosine of an angle is defined as the ratio of the length of the adjacent side to the length of the hypotenuse in a right triangle.
In the case of cos 30, we consider a right triangle where one of the angles is 30 degrees.
By convention, in a right triangle, the side adjacent to an angle is the side that is adjacent to the angle and not the hypotenuse.
In this case, let's assume that the adjacent side is of length x, and the hypotenuse is of length 1.
Now, we can use the cosine definition: cos 30 = x/1 = x.
To find the value of cos 30, we need to determine the length of the adjacent side of the right triangle.
Since we know that the angle is 30 degrees, we can use the properties of a 30-60-90 triangle.
In a 30-60-90 triangle, the sides are in the ratio 1:sqrt(3):2.
The length of the adjacent side in a 30-60-90 triangle is half the length of the hypotenuse.
Therefore, the length of the adjacent side is 1/2.
Substituting this value into our equation, we find that cos 30 = 1/2.
Therefore, the value of cos 30 is A. 1/2.