View more at http://www.MathAndScience.com. In this lesson, we will learn fundamentally what the sine function and cosine function represent. We will learn that the sine function, also written as sin or sin(x), reduces the hypotenuse of a right triangle to the projection onto the y-axis. Similarly, the cosine function, also written as cos or cos(x), reduces the hypotenuse of a right triangle to the projection onto the x-axis. Thinking of sine and cosine as projection functions like this helps us to visualize exactly what the sin(x) and cos(x) functions do. This has applications in math, engineering, physics, and all branches of science.