Finish the proof.
cos 2x = -2 sin 2x. (Use the double-angle identity.)
A. 
cos 2x = 
(cos2 x - sin2 x) = 
[(cos x)(cos x) - (sin x)(sin x)]
= (-sin x)(cos x) + (cos x)(-sin x) - (cos x)(sin x) + (sin x)(cos x) = -2 sin x cos x = -2 sin 2x
B. 
cos 2x = 
(cos2 x - sin2 x) = 
[(cos x)(cos x) - (sin x)(sin x)]
= (-sin x)(cos x) + (cos x)(-cos x) - (cos x)(sin x) - (sin x)(sin x)
= -2 sin x cos x - cos2 x - sin2 x = -2 sin 2x
C. 
cos 2x = 
(cos2 x - sin2 x) = 
[(cos x)(cos x) - (sin x)(sin x)]
= (-sin x)(cos x) + (cos x)(-sin x) - (cos x)(sin x) - (sin x)(cos x) = -4 sin x cos x = -2 sin 2x
D. 
cos 2x = 
(cos2 x - sin2 x) = 
[(cos x)(cos x) - (sin x)(sin x)] = 2 cos x - 2 sin x
= -4 sin x cos x = -2 sin 2x
Answer: C
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A. - 
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x
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1655
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