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B=mi*I/(4R*pi)*(cos(a)-cos(b)) or B=mi*I/(4R*pi)*(cos(a)+cos(b))?

For a infinitely long conductor, the law is:

B=mi*I/(2R*pi) because a=0 and b=pi. So I would say that the correct expression is the one where the cos are subtracted, but I was solving a problem where a point is in the center of a square-like conductor and they used the formula with the addition of cos to get the magnetic field at that point, as with the subtraction the answer would be zero. What equation is the correct one, I am really confused?