What is the value of cos(Ï/4-x) cos(Ï/4-y)-sinâ¡ã(Ï/4-x) sin(Ï/4-y)ã?
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cosAcosB+sinAsinB=cos(A+B)
so the value is cos(1/4-x+1/4-y)
=cos(2/4-x-y)
=cos(1/2-x-y)
so the value is cos(1/4-x+1/4-y)
=cos(2/4-x-y)
=cos(1/2-x-y)
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