Talk:Double Angle Formulas/Cosine

In high school in my country, Corollaries 1 and 2 are known as Carnot's formulas. Can't find a real reference, only things like this: http://koendenaeghel.be/Zesde%20jaar%206u/formularium.pdf --barto (talk) 11:37, 8 September 2017 (EDT)