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A student claims: “The equation \(\ln(x^2)=2\ln(x)\) is valid for every \(x\in\mathbb{R}\setminus\{0\}\) because the exponent can be moved in front of the logarithm.”
Evaluate the claim by comparing the domains of the two sides.
Hints
- When is a natural logarithm defined over the real numbers?
- Test a negative value of \(x\).
- Compare the domains of both sides before applying a logarithm property.
- What happens to a negative number when it is squared?
Solution
1. The left side, \(\ln(x^2)\), is defined when \(x^2>0\), which means \(x\ne0\).
2. The right side, \(2\ln(x)\), is defined only when \(x>0\).
3. The domains are not the same. For example, when \(x=-2\), \(\ln(x^2)=\ln(4)\) is defined, but \(2\ln(-2)\) is not real.
4. Therefore, the student's statement is false. The identity \(\ln(x^2)=2\ln(x)\) is valid only for \(x>0\). For all \(x\ne0\), the correct identity is \(\ln(x^2)=2\ln|x|\).
Answer
The claim is false. The left side is defined for \(x\ne0\), while the right side is defined only for \(x>0\).
