formula of principal stresses:

(Sx+Sy)/2 +- sqrt( [(Sx+Sy)/2]^2 + T^2 )

I got Sx, Sy, and T in a complex format: a+bi

I would like to know what is the correct way to compute the principal stresses:


--- perform the calculations in a complex format and in the end compute the magnitude, or

--- compute magnitude of Sx, Sy and T and replace in the formula of the principal stresses.

Thank you