Đáp án:
k) \(x = \dfrac{3}{2}\)
Giải thích các bước giải:
\(\begin{array}{l}
a)\Delta ' = 9 - 5.\left( { - 8} \right) = 49\\
\to \left[ \begin{array}{l}
x = \dfrac{{3 + 7}}{5} = 2\\
x = \dfrac{{3 - 7}}{5} = - \dfrac{4}{5}
\end{array} \right.\\
b)\Delta ' = 9 - 1.\left( { - 3} \right) = 12\\
\to \left[ \begin{array}{l}
x = - 3 + 2\sqrt 3 \\
x = - 3 - 2\sqrt 3
\end{array} \right.\\
c)\Delta = 9 - 4.2.\left( { - 5} \right) = 49\\
\to \left[ \begin{array}{l}
x = \dfrac{{ - 3 + 7}}{4} = 1\\
x = \dfrac{{ - 3 - 7}}{4} = - \dfrac{5}{2}
\end{array} \right.\\
d)\Delta ' = 4 - 2.\left( { - 3} \right) = 10\\
\to \left[ \begin{array}{l}
x = \dfrac{{ - 2 + \sqrt {10} }}{2}\\
x = \dfrac{{ - 2 - \sqrt {10} }}{2}
\end{array} \right.\\
e)\Delta = 25 - 4.3 = 13\\
\to \left[ \begin{array}{l}
x = \dfrac{{5 + \sqrt {13} }}{2}\\
x = \dfrac{{5 - \sqrt {13} }}{2}
\end{array} \right.\\
g)\Delta = 9 - 4.\left( { - 3} \right) = 21\\
\to \left[ \begin{array}{l}
x = \dfrac{{3 + \sqrt {21} }}{2}\\
x = \dfrac{{3 - \sqrt {21} }}{2}
\end{array} \right.\\
h)\Delta = 9 - 4.2.\left( { - 2} \right) = 25\\
\to \left[ \begin{array}{l}
x = \dfrac{{3 + 5}}{2} = 4\\
x = \dfrac{{3 - 5}}{2} = - 1
\end{array} \right.\\
i){x^2} - 6x + 9 = 0\\
\to {\left( {x - 3} \right)^2} = 0\\
\to x - 3 = 0\\
\to x = 3\\
k)4{x^2} - 12x + 9 = 0\\
\to {\left( {2x - 3} \right)^2} = 0\\
\to 2x - 3 = 0\\
\to x = \dfrac{3}{2}
\end{array}\)
