Giải thích các bước giải:
\(\begin{array}{l}
B20:\\
a.\left( {{x^2} + 1} \right)\left( {7 - x} \right)\left( {7 + x} \right) = 0\\
\to \left[ \begin{array}{l}
x = 7\\
x = - 7
\end{array} \right.\left( {do:{x^2} + 1 > 0\forall x \in R} \right)\\
b.Pt \to 6 - 2\left| x \right| = 0\left( {do:\left| x \right| + 2 > 0\forall x} \right)\\
\to 3 = \left| x \right|\\
\to x = \pm 3\\
B21:\\
a.\left( {x - 2} \right)\left( {7 - x} \right) > 0\\
\to \left[ \begin{array}{l}
\left\{ \begin{array}{l}
x - 2 > 0\\
7 - x > 0
\end{array} \right.\\
\left\{ \begin{array}{l}
x - 2 < 0\\
7 - x < 0
\end{array} \right.
\end{array} \right. \to \left[ \begin{array}{l}
2 < x < 7\\
\left\{ \begin{array}{l}
x < 2\\
x > 7
\end{array} \right.\left( {vô lý} \right)
\end{array} \right.
\end{array}\)
\(\begin{array}{l}
KL:2 < x < 7\\
b.\left( {x - \sqrt {13} } \right)\left( {x + \sqrt {13} } \right)\left( {x - \sqrt {17} } \right)\left( {x + \sqrt {17} } \right) < 0\\
Xét:\left( {x - \sqrt {13} } \right)\left( {x + \sqrt {13} } \right)\left( {x - \sqrt {17} } \right)\left( {x + \sqrt {17} } \right) = 0\\
\to \left[ \begin{array}{l}
x = \sqrt {13} \\
x = - \sqrt {13} \\
x = \sqrt {17} \\
x = - \sqrt {17}
\end{array} \right.
\end{array}\)
BXD:
x -∞ -√17 -√13 √13 √17 +∞
f(x) + 0 - 0 + 0 - 0 +
\(KL:x \in \left( { - \sqrt {17} ; - \sqrt {13} } \right) \cup \left( {\sqrt {13} ;\sqrt {17} } \right)\)
