Ответ:

  1. \(x^2-6x-2=(x-3)^2-11\).
  2. \(x^2+5x+20=\left(x+\frac52\right)^2+\frac{55}{4}\).
  3. \(2x^2-4x+10=2(x^2-2x+5)=2(x-1)^2+8\).
  4. \(\frac12x^2+x-6=\frac12(x^2+2x-12)=\frac12(x+1)^2-\frac{13}{2}\).