\[\boxed{\text{315.}\text{\ }\text{еуроки}\text{-}\text{ответы}\text{\ }\text{на}\text{\ }\text{пятёрку}}\]
Пояснение.
Решение.
\[Перенесем\ числа\ в\ левую\ часть\ \]
\[уравнения,\ меняя\ знак\ на\ \]
\[противоположный.\]
\[При\ переносе\ буквенной\ части,\ \]
\[тоже\ меняем\ знак\ на\ \]
\[противоположный.\]
\[\textbf{а)}\ 80 + y^{2} = 81\]
\[y^{2} = 81 - 80\]
\[y^{2} = 1\]
\[y = \sqrt{1}\]
\[y = \pm 1.\]
\[\textbf{б)}\ 19 + c^{2} = 10\]
\[c^{2} = 10 - 19\]
\[c = - 9 < 0\]
\(c = \varnothing\).
\[\textbf{в)}\ 20 - b^{2} = - 5\]
\[b^{2} = 20 + 5\]
\[b^{2} = 25\]
\[b = \sqrt{25}\]
\[b = \pm 5.\]
\[\textbf{г)}\ 3x^{2} = 1,47\]
\[x^{2} = 1,47\ :3\]
\[x^{2} = 0,9\]
\[x = \sqrt{0,49}\]
\[x = \pm 0,7.\]
\[\textbf{д)}\frac{1}{4}a^{2} = 10\]
\[a^{2} = 40\]
\[a = \sqrt{40}\]
\[a = \pm 2\sqrt{10}.\]
\[\textbf{е)} - 5y^{2} = 1,8\]
\[y^{2} = 1,8\ :( - 5) < 0\]
\[y = \varnothing.\]