This series of posts explores the fundamental concepts, applications, and advanced topics related to cosine in mathematics. Starting with an introduction to cosine and its significance in mathematics, we then dive into the definition of the cosine function and its connection to right triangles and the unit circle. We also cover graphing cosine functions and understanding how different parameters affect the graph. Additionally, we explore real-life applications of cosine and how to solve trigonometric equations using cosine. Finally, we delve into advanced topics such as the cosine rule, cosine similarity, and Fourier series to showcase the broader scope and applications of cosine in various fields.