help15

I think you must mean 2*cos(pi/3 + A) which is cos(A) - sqrt(3)*sin(A)

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cos(x + y) = cos(x)*cos(y) - sin(x)*sin(y)

 

so  cos(pi/3  + A) = cos(pi/3)*cos(A) - sin(pi/3)*sin(A)

 

cos(pi/3) = 1/2  and sin(pi/3) = sqrt(3)/2      These are standard values that are worth remembering.

 

Hence cos(pi/3 + A)  = (1/2)cos(A) - (sqrt(3)/2)sin(A)

 

Multiply by the 2 to get 2cos(pi/3 + A) = cos(A) - sqrt(3)sin(A)