Question 730054: Which is the value of cos 2theta if cos theta= 5/13 and 0 degrees< theta < 90 degrees?
Answer by stanbon(75887)   (Show Source):
You can put this solution on YOUR website! Which is the value of cos(2theta) if cos theta= 5/13 and 0 degrees< theta < 90 degrees?
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Theta is in QI where x,y,and r are positive.
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cos(theta) = 5/13 implies that x = 5 and r = 13
Therefore y = sqrt[13^2-5^2] = 12
So sin(theta) = y/r = 12/13
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cos(2theta) = cos^2(theta)-sin^2(theta) = (5/13)^2-(12/13)^2 = -0.7041
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Cheers,
Stan H.
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