I was playing around with this function:
$$f(x)=\log_{\frac{1}{2}}(x^2-x-6)-\log_{\frac{1}{2}}\frac{x+2}{x-3}+4$$
I tried to find values of $x$ such that $f(x)\leq 0$. But for some reason, the answer I obtain by solving this inequality analytically does not match the answer I find by plotting this function. Here are my steps:
First I rewrite both logarithms using their properties
$$\log_{\frac{1}{2}}((x+2)(x-3))-\log_{\frac{1}{2}}(x+2)+\log_{\frac{1}{2}}(x-3)+4\leq0$$
$$\log_{\frac{1}{2}}(x+2)+\log_{\frac{1}{2}}(x-3)-\log_{\frac{1}{2}}(x+2)+\log_{\frac{1}{2}}(x-3)+4\leq0$$
$$2\log_{\frac{1}{2}}(x-3)+4\leq0\implies 2\log_{\frac{1}{2}}(x-3)\leq -4$$
I rewrite the base of the logarithm
$$-\log_{2}(x-3)\leq -2\implies \log_{2}(x-3)\geq 2$$
$$x-3\geq4\implies x\geq7$$
Then I find the intersection of my solution with the condition that arguments of logarithms in the initial function have to be both positive, which turns out to be $x\in(-\infty;-2)\cup(3;+\infty)$. The result of this action is $x\geq 7$.
However, when I plot $y=f(x)$, I see this solution: $x\in(-\infty;-2)\cup[7;+\infty)$.
I'm not sure I see what I am somehow missing in my solution. Why are the answers different? What is my solution missing?
Thank you in advance :)
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Tom
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2The issue is that $\log x y$ is not always $\log x+\log y$, but sometimes is $\log(-x)+\log(-y)$. You need to repeat the steps for the case $x<-2$. – Andrei Nov 29 '23 at 15:37
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Oh, I see. Thank you for your help! – Tom Nov 29 '23 at 15:39