10.5 Practice Problems

Problem 10.1. Solve

(a).
\(\left |3x-2\right |>2\)
(b).
\(\frac {2}{x-3}<\frac {3}{2x-1}\)

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Solution. (a). A “greater than” absolute value gives two separate pieces: \[3x-2<-2 \quad \text {or}\quad 3x-2>2,\] that is \(3x<0\) or \(3x>4\), so \[x<0 \quad \text {or}\quad x>\frac {4}{3}, \quad \text {i.e. } (-\infty ,0)\cup \left (\tfrac {4}{3},\infty \right ).\]

(b). Never cross-multiply: both denominators change sign. Move everything to one side, \[\frac {2}{x-3}-\frac {3}{2x-1}<0 \implies \frac {2(2x-1)-3(x-3)}{(x-3)(2x-1)}<0 \implies \frac {x+7}{(x-3)(2x-1)}<0 .\] The critical values are \(x=-7\), \(x=\frac {1}{2}\) and \(x=3\). Testing the sign of the whole expression on each interval between them:

interval \((-\infty ,-7)\) \(\left (-7,\tfrac {1}{2}\right )\) \(\left (\tfrac {1}{2},3\right )\) \((3,\infty )\)
\(x+7\) \(-\) \(+\) \(+\) \(+\)
\(x-3\) \(-\) \(-\) \(-\) \(+\)
\(2x-1\) \(-\) \(-\) \(+\) \(+\)
quotient \(-\) \(+\) \(-\) \(+\)

The expression is negative on \((-\infty ,-7)\) and on \(\left (\frac {1}{2},3\right )\), and all three critical values are excluded — \(-7\) because the inequality is strict, the other two because the expression is undefined there. Hence \[(-\infty ,-7)\cup \left (\tfrac {1}{2},3\right ).\]

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