Answer
All the three formulas are equivalent.
Work Step by Step
We know that $\cos 2x=\cos^2 x-\sin^2 x$ ...(1)
Here, we find two equations obtained form the aboive equation (1).
1) $\cos 2x=1-2 \sin^2 x \implies \cos 2x=\sin^2 x+\cos^2 x-2 \sin^2 x$
and $\cos 2x=1-2 \sin^2 x \implies \cos 2x=\cos^2 x- \sin^2 x$
2) $\cos 2x=2 \cos^2 x -1 \implies \cos 2x=2 \cos^2 x-(\sin^2 x+cos^2 x)$
and $\cos 2x=\cos^2 x- \sin^2 x$
Hence, it has been verified that all the three formulas are equivalent.
