Cosine Function

The cosine function, is one of the three main trigonometry functions. The cosine of the angle theta is the ratio of the adjacent side over the hypotenuse. This ratio is the same on any size circle. On the unit circle the hypotenuse is equal to one, so the cosine value is equal to x variable in the Cartesian plane. The cosine function is the reciprocal function of the secant function. The cosine of 0 degrees is equal to one. As the theta angle goes from 0 degrees to 90 degrees the value of its cosine moves from one to zero in the first quadrant of the coordinate system. As the theta angle hits 90 degrees the value of the cosine reaches zero. The angle then extends into the second quadrant and the value of the cosine continues decreasing. Here the value is negative because the x variable is negative. The hypotenuse is positive, always remaining equal to one. As the theta angle goes through 180 degrees it reaches negative one and then enters the third quadrant. Finally the value starts to increase moving from negative one up to zero at a theta angle equal to 270 degrees. As the theta angle enters the fourth quadrant the value of the cosine increases from zero back to reach one again at 360 degrees. Any angle beyond this repeats the cycle. The domain of the cosine function is the set of real number. It represents the angle input into the function and can be expressed in degrees or radians. The range of the cosine function is limited from negative one to positive one.