\[\boxed{\text{154.}\text{\ }\text{ОК\ ГДЗ\ -\ домашка\ на\ 5}}\]
\[\textbf{а)}\ y = - \frac{1}{2}x^{2} + 5\]
\[\textbf{б)}\ y = x² - 4x\]
\[\textbf{в)}\ y = - x^{2} + 6x - 9\]