Skrytý text:Since

, you can consider

instead. If you write it out as a function of real and imaginary part of z, you will se, the problem can be reduced to finding minimum of

. Clearly, the minimum will be somewhere between 1/2 and 3/4. And since Q is continuous and having derivative of any order it's sufficient to investigate its local extremes (it's just a parabola, actually).

and so the minimum of Q is obtained for

So the minimum value of P(x) will be obtained at

and I can't be bothered to calculate it.