tyjae1601 tyjae1601

07-06-2019
Mathematics

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If cos x = −6/7, and 90° < x < 180°, what is cos⁡(2x)?

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LammettHash LammettHash

07-06-2019

Recall the double angle identity:

[tex]\cos2x=\cos^2x-\sin^2x=2\cos^2x-1[/tex]

Then

[tex]\cos2x=2\left(-\dfrac67\right)^2-1=\dfrac{23}{49}[/tex]

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