Ans: All the periodic and non-periodic functions have their own importance in the world of trigonometry. Cosine is also a periodic function and has a huge application in mathematics. With the help of the unit circle, we can understand the cosine function. It is the simplest way to understand the cosine function. Firstly, draw a unit circle on the coordinate with its centre at an angle theta. The right side has a positive x-axis and the left side has a negative x-axis. The positive y-axis is (0, sineθ). The positive x-axis is (cosθ, 0). The hypotenuse is (cosθ, sineθ).