SOLUTION: If sin A = -4/5 with A in QIV, and sin B = 3/5 with B in QI, find cos (A+B).

Question 960752: If sin A = -4/5 with A in QIV, and sin B = 3/5 with B in QI, find cos (A+B).
Answer by stanbon(75887) (Show Source): You can put this solution on YOUR website!
If sin A = -4/5 with A in QIV
then cos(A) = sqrt[25-16]/5 = 3/5
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If sin B = 3/5 with B in QI
then cos(B) = -sqrt[25-9]/5 = -4/5
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Find cos (A+B) = cos(A)cos(B)-sin(A)*sin(B)
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= (3/5)(-4/5)-(-4/5)(3/5)
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= 0
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Cheers,
Stan H.
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