10 Secret Trig Functions Your Math Teachers Never Taught YouHaversine? Exsecant? An introduction to obsolete trig functions and why we don’t use them anymore Scientific American Blog NetworkEvelyn Lamb [https://blogs.scientificamerican.com/roots-of-unity/10-secret-trig-functions-your-math-teachers-never-taught-you/] Excerpt from https://blogs.scientificamerican.com/roots-of-unity/10-secret-trig-functions-your-math-teachers-never-taught-you/