\[\boxed{\text{859.}\text{\ }еуроки - ответы\ на\ пятёрку}\]
\[\textbf{а)}\ x² - 16x + 64 = (x - 8)^{2} \geq 0\]
\[\textbf{б)}\ 16 + 8x + x² = (4 + x)^{2} \geq 0\]
\[\textbf{в)} - x^{2} - 4x - 4 = - \left( x^{2} + 4x + 4 \right) = - (x + 2)^{2} \leq 0\]
\[\textbf{г)} - x^{2} + 18x - 81 = - \left( x^{2} - 18x + 81 \right) = - (x - 9)^{2} \leq 0\ \]