Bài tập 4.27 trang 206 SBT Toán 12

a) 

\(\begin{array}{l}
2{x^2} + 3x + 4 = 0,\Delta  =  - 23,\sqrt \Delta   = \sqrt {23} i\\
 \Leftrightarrow \left[ \begin{array}{l}
x = \frac{{ - 3 + i\sqrt {23} }}{4}\\
x = \frac{{ - 3 - i\sqrt {23} }}{4}
\end{array} \right.
\end{array}\)

b)

\(\begin{array}{l}
3{x^2} + 2x + 7 = 0,\Delta ' =  - 20,\sqrt {\Delta '}  = 2\sqrt 5 i\\
 \Leftrightarrow \left[ \begin{array}{l}
x = \frac{{ - 1 + 2i\sqrt 5 }}{3}\\
x = \frac{{ - 1 - 2i\sqrt 5 }}{3}
\end{array} \right.
\end{array}\)

c)

\(\begin{array}{l}
2{x^4} + 3{x^2} - 5 = 0,\Delta  = 49,\sqrt \Delta   = 7\\
 \Leftrightarrow \left[ \begin{array}{l}
{x^2} =  - \frac{5}{2}\\
{x^2} = 1
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
x =  \pm i\sqrt {\frac{5}{2}} \\
x =  \pm 1
\end{array} \right.
\end{array}\)

-- Mod Toán 12 HỌC247