SOLUTION: If sin x = 1/√5 and cos is less than 0, find cos 2x.

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Question 702689: If sin x = 1/√5 and cos is less than 0, find cos 2x.
Answer by sachi(548) About Me  (Show Source):
You can put this solution on YOUR website!
If sin x=1/√5,
and cos x =sqrt(1-sin^2x)=sqrt(1-1/5)=+/- 2/√5 is less than 0
so cos x =- 2/√5
cos 2x=cos^2x-sin^2x=[- 2/√5]^2-[1/√5]^2=4/5-1/5=3/5
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