A proof not requiring separate cases is as follows: Any number is always less than or equal to its own absolute value, so and Adding these inequalities together, the sum has the form for and But this always implies , or, expanded, James Stewart (2008). Essential Calculus. Thomson Brooks/Cole. p. A10. ISBN978-0-495-10860-3.
Schubert, Erich (2021). A Triangle Inequality for Cosine Similarity. International Conference on Similarity Search and Applications. Dortmund: Springer. arXiv:2107.04071. doi:10.1007/978-3-030-89657-7_3.
Schubert, Erich (2021). A Triangle Inequality for Cosine Similarity. International Conference on Similarity Search and Applications. Dortmund: Springer. arXiv:2107.04071. doi:10.1007/978-3-030-89657-7_3.