Let \(g(x)=\sqrt{\ln(x^2-3)}\), with its maximal domain.
a) Find the domain.
b) Find all \(x\) in the domain for which \(g(x)=\sqrt{\ln(13)}\).
Hints
- The logarithm must be defined and the square-root input must be nonnegative.
- Determine when a natural logarithm is at least zero.
- Square both sides and compare logarithm inputs.
- Include both roots of the resulting quadratic equation.
Solution
1. The radicand must satisfy \(\ln(x^2-3)\geq0\). This condition also ensures that the logarithm input is positive.
2. Since \(\ln(u)\geq0\) exactly when \(u\geq1\), \(x^2-3\geq1\). Thus, \(x^2\geq4\), so the domain is \((-\infty, -2]\cup[2, \infty)\).
3. Set \(\sqrt{\ln(x^2-3)}=\sqrt{\ln(13)}\). Squaring gives \(\ln(x^2-3)=\ln(13)\).
4. Therefore, \(x^2-3=13\), so \(x^2=16\) and \(x=-4\) or \(x=4\). Both values lie in the domain.
Answer
a) \((-\infty, -2]\cup[2, \infty)\)
b) \(x=-4\) and \(x=4\)