What is cos X?

Answer 1

At the most basic level, cos(x) represents the cosine of an angle x, a trigonometric function. In a right angled triangle, where one of the angles is x, cos(x) is the ratio of the lengths of the short side next to the angle x (the adjacent side) and the hypotenuse (the side opposite the right angle).

Soon afterwards, you learn that this measure is a characteristic of the angle x, not of the triangle, so you do not actually need a triangle! Any angle x has a cos(x) associated with it, whether it is in a triangle, in another shape or simply a free-standing angle.

Later still, you learn that sin(x) is an infinite series of the form:

cos(x) = 1 - x2/2! + x4/4! - x6/6! + ... where

x is the angle measured in radians,

and

n! [n factorial] is n*(n-1)*(n-2)*...*3*2*1

Alternate answer:

X is a variable representing the measure of an angle.

Cosine, abbreviated cos, is a ratio of two sides of a right triangle containing the angle X. but not any 2 sides, 2 specific sides of the triangle. (ratio means a fraction, which is usually divided to give a decimal answer)

One of those sides is called the adjacent and the other is called the hypotenuse. The third side is called the opposite.

Cos X = Adjacent side / hypotenuse

Answer 2

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