SOLUTION: Pleas help me solve this equation: find cos(s+t) given that cos s=1/3, with s in quadrant I , and sin t=-1/2, with t in quadrant IV

Question 865799: Pleas help me solve this equation: find cos(s+t) given that cos s=1/3, with s in quadrant I , and sin t=-1/2, with t in quadrant IV
Answer by stanbon(75887) (Show Source): You can put this solution on YOUR website!
find cos(s+t) given that cos s=1/3, with s in quadrant I , and sin t=-1/2, with t in quadrant IV
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Formula:: cos(s+t) = cos(s)cos(t)-sin(s)sin(t)
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Since cos(s) = 1/3 in QI, x = 1 and r = 3
Then y = sqrt(3^2-1) = sqrt(8)
Therefore sin(s) = y/r = sqrt(8)/3
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Since sin(t) = -1/2 in QIV, y = -1 and r = 2
Then x = sqrt(2^2-1) = sqrt(3)
Therefore cos(t) = x/r = sqrt3)/2
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Plug those into the Formula
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Cheers,
Stan H.
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