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Sagot :
Answers:
- cos(2theta) = -119/169
- cos(theta) = -5/13
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The given info is:
- sin(theta) = 12/13
- theta is in quadrant II
From that, we can use the pythagorean trig identity to find that cos(theta) = -5/13. Keep in mind that cosine is negative in quadrant II.
Now use the trig identity below to compute cos(2theta)
[tex]\cos(2\theta) = \cos^2(\theta)-\sin^2(\theta)\\\\\cos(2\theta) = \left(\frac{-5}{13}\right)^2-\left(\frac{12}{13}\right)^2\\\\\cos(2\theta) = \frac{25}{169}-\frac{144}{169}\\\\\cos(2\theta) = \frac{25-144}{169}\\\\\cos(2\theta) = -\frac{119}{169}\\\\[/tex]
Other options you could use are these identities
[tex]\cos(2\theta) = 2\cos^2(\theta)-1\\\\[/tex]
or
[tex]\cos(2\theta) = 1-2\sin^2(\theta)\\\\[/tex]
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