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Pls help with the 3rd one |
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Answer» Step-by-step explanation: COS(π/4 - x)Cos(π/4 - y) - Sin(π/4 - x)Sin(π/4 - y) //We KNOW that CosACosB - SinASinB = Cos(A+B) => Cos(π/4 - x + π/4 - y) => Cos(2π/4 - (x+y)) => Cos(π/2 - (x+y)) => Sin(x+y) (∵ Cos(90-θ) = Sinθ) |
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