Even though you may naively imagine that trigonometric functions are important only for geometry, they are actually very important and widely used in mathematics. For example, trigonometric functions are important in describing waves in physics. As quantum mechanics is about particles and waves, knowledge of trigonometric functions is essential in understanding quantum mechanics. One of the purposes of the article “Rotation in Cartesian coordinates” is to serve as a basis to make an interesting comparison with the Lorentz transformation in my article on that topic in the section “For students learning high school physics.”