In my Calculus book I have one statement:
$$\cos(2x) = \cos^2(x)-\sin^2(x)$$
and a couple of rows down another statement is:
$$ \cos(2x) = 2 \cos^2(x) - 1 = 1 - 2 \sin^2(x).$$
Now when trying to memorize these statements i write (over and over to see if i know it by heart)
$$ \cos(2x) = \cos^2(x) - \sin^2(x) = 2 \cos^2(x)-1 = 1 - 2\sin^2(x). $$
Is this an legitimate thing to write am I misusing the equal sign?