only true if theta = 45 degrees as far as I know
identity:
cos 2t = cos^2 t - sin^2 t
on the right we have
.5 [ cos t/ sin t - sin t/cos t ]
= .5 [ cos^2 t - sin^2 t] /(sin t cos t)
that is only equal to the left side if
sin t cos t = .5
which is only true if t = 45 degrees or 225 degrees
PROVE: Cos 2theta = .5 (cot theta - tan theta)
Please help me
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