1.

Pls help with the 3rd one

Answer»

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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