Выделите квадрат двучлена х²-8х-1 4х²-20х+25

Решите задачу:

$(a\pm b)^2=a^2\pm 2ab+b^2\quad \Rightarrow \quad a^2\pm 2ab=(a\pm b)^2-b^2\\\\ili\qquad x^2\pm px=(x\pm \frac{p}{2})^2-(\frac{p}{2})^2\\\\\\\underbrace {x^2-8x}-1=\underbrace {(x-4)^2-4^2}_{x^2-8x}-1=(x-4)^2-16-1=(x-4)^2-17\\\\\\4x^2-20x+25=4\cdot \Big (\underbrace {x^2-5x}+\frac{25}{4}\Big )=4\cdot \Big (\underbrace {(x-\frac{5}{2})^2-(\frac{5}{2})^2}_{x^2-5x}+\frac{25}{4}\Big )=\\\\=4\cdot \Big ((x-2,5)^2-\frac{25}{4}+\frac{25}{4}\Big )=4\cdot (x-2,5 )^2\\\\ili\qquad 4x^2-20x+25=(2x-5)^2$