Can someone help explain this trigonometry equation to me with a detailed development?
sin^{2}a - cos^{2}a = 1-2 x cos^{2}a
thank you!
One of the most basic identities to memorize because you will use it frequently is: $\displaystyle \sin^2a + \cos^2a = 1$. Rearrange this to get the $\displaystyle \sin^2a$ by itself, and then substitute for the $\displaystyle \sin^2a$ term in your equation - what do you get? By the way, you could also have replaced the $\displaystyle \cos^2a$ term in your equation to get a different form of the same equation, all of which are equal to $\displaystyle \cos(2a)$.