\[\boxed{\text{895.}\text{\ }еуроки - ответы\ на\ пятёрку}\]
\[\textbf{а)}\ (a + b)^{2} - 4ab = (a - b)²\]
\[a^{2} + 2ab + b^{2} - 4ab - a^{2} - 2ab + b^{2} = (a - b)²\]
\[(a - b)^{2} = (a - b)^{2} \Longrightarrow верно.\]
\[\textbf{б)}\ (a - b)^{2} + 4ab = (a + b)²\]
\[a^{2} - 2ab + b^{2} + 4ab = (a + b)²\]
\[a^{2} + 2ab + b^{2} = (a + b)²\]
\[(a + b)^{2} = (a + b)^{2} \Longrightarrow верно.\]
\[\textbf{в)}\ (x + 3)^{3} + (x - 3)^{3} = 2x³ + 54x\]
\[x³ + 9x² + 27x + 27 + x³ - 9x^{2} + 27x - 27 = 2x³ + 54x\]
\[2x^{3} + 54x = 2x^{3} + 54x \Longrightarrow верно.\]