Question 810786: Find cos(2 theta)
Given : tan theta = 12/5, theta is in quadrant 3
Answer by stanbon(75887)   (Show Source):
You can put this solution on YOUR website! Find cos(2 theta)
Given : tan theta = 12/5, theta is in quadrant 3
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Note: Both x and y are negative in QIII
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Since tan = y/x = 12/5, y = -12 and x = -5
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Then r = sqrt[12^2+5^2] = sqrt[169] = 13
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cos = x/r = -5/13
sin = y/4 = -12/13
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Your Problem::
cos(2 t) = cos^2(t)-sin^2(t) = (5/13)^2 - (12/13)^2 = (25-144)/169 = -119/169
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Cheers,
Stan H.
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