The concept of cosine is of exclusive use within the trigonometry, as the branch of mathematics that deals with the study of trigonometric ratios, how to be: cosine, sine, tangent, cotangent, secant, and cosecant.
Meanwhile, at the request of a right triangle, the cosine of an acute angle will be defined as the ratio between one of the legs adjacent to the mentioned angle and the hypotenuse. The cosine is abbreviated with the acronym of cos. A leg is that smaller side, there are two, of a right triangle, which are the ones that together make up the right angle. Meanwhile, the larger side is designated as the hypotenuse, which will be the one that opposes the right angle.
It should be noted that the relationship that opposes cosine is the secant, the trigonometric ratios are the cosine, the sine and the tangent and the inverse trigonometric ratios are the mentioned secant, cotangent and cosecant.
Trigonometry will be applied wherever it is necessary to obtain precise measurements of something, it is applied in most branches of mathematics and also in other disciplines, such is the case of astronomy to measure the closest stars, the distances of geographical points, and in navigation systems that involve satellites. The geometry of space also makes use of trigonometry.
The study of the sides of triangles dates back thousands of years, to Babylonian culture to be more precise. The astronomers of these times kept very precise and detailed accounts of the rising and setting of the stars as well as the movements of the planets and the eclipses of the sun and moon. Meanwhile, all this would have been impossible to determine if the angular distance measured on the planet was not known.