Đáp án:
b) \(\begin{array}{l}
f\left( x \right) > 0 \Leftrightarrow x \in \left( { - 3; + \infty } \right)\\
f\left( x \right) < 0 \Leftrightarrow x \in \left( { - \infty ; - 3} \right)
\end{array}\)
Giải thích các bước giải:
a) BXD:
x -∞ 3 +∞
f(x) + 0 -
\(\begin{array}{l}
KL:f\left( x \right) > 0 \Leftrightarrow x \in \left( { - \infty ;3} \right)\\
f\left( x \right) < 0 \Leftrightarrow x \in \left( {3; + \infty } \right)
\end{array}\)
b) BXD:
x -∞ -3 +∞
f(x) - 0 +
\(\begin{array}{l}
KL:f\left( x \right) > 0 \Leftrightarrow x \in \left( { - 3; + \infty } \right)\\
f\left( x \right) < 0 \Leftrightarrow x \in \left( { - \infty ; - 3} \right)
\end{array}\)
c) BXD:
x -∞ -2 1 +∞
f(x) - 0 + 0 -
\(\begin{array}{l}
KL:f\left( x \right) > 0 \Leftrightarrow x \in \left( { - 2;1} \right)\\
f\left( x \right) < 0 \Leftrightarrow x \in \left( { - \infty ; - 2} \right) \cup \left( {1; + \infty } \right)
\end{array}\)
d) \(DK:x \ne 3\)
BXD:
x -∞ 1/2 3 +∞
f(x) + 0 - // +
