How to find the domain of the function $\sqrt{ \log_{\frac{1}{2}} x}$ ?
Domain Problem: $\sqrt{ \log_{\frac{1}{2}} x}$
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algebra-precalculus
logarithms
1 Answers
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Assuming the result is real, we must have $\log _{\frac{1}{2}}x\geq 0$.
Since $\log _{\frac{1}{2}}x\geq 0\Leftrightarrow 0 Added 2: $\log _{\frac{1}{2}}x=\frac{\log x}{\log \frac{1}{2}}=\frac{\log x}{\log
1-\log 2}=\frac{\log x}{0-\log 2}=-\frac{\log x}{\log 2}$ $\log _{\frac{1}{2}}x\geq 0\Leftrightarrow-\frac{\log x}{\log 2}\geq 0\Leftrightarrow\log x\le 0\Leftrightarrow 0 Added: plot of $\log_{\frac{1}{2}}x$ (green) and $\sqrt{\log_{\frac{1}{2}}x}$ (blue).
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0I am new to logarithms,rather learned it recently,I understood ur explanation.I want you ask you could you please explain this step: "we must have $\log _{\frac{1}{2}}x\geq 0$".What is the rule exactly for logarithms ? – 2010-11-20
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0Also why I am getting negative values in `Table[Log[.5, x], {x, 0, 100}]` ... I am confused :( – 2010-11-20
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0"we must have $\log_\frac{1}{2}x\ge 0$" because the radicand has to be greater or equal to zero, otherwise you would have a complex result. And I assumed that it is real. For instance $\sqrt{\log _{\frac{1}{2}}4}=\sqrt{-2}=i\sqrt{2}$. – 2010-11-20
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0The function $\log_{\frac{1}{2}}x$ is negative for $x\gt 1$, positive for $x\lt 1$ and equal to zero for $x=1$. – 2010-11-20
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0@Deb: it might be instructive to note that $\log_{\frac12}x=-\log_2 x$ (prove this!); now note that this is within a square root, and then look at Américo's answer again. – 2010-11-20
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1Thanks, now I understand! @J.M: Proof is like this : $\log _{\frac{1}{2}}x=\frac{\log x}{\log \frac{1}{2}}=\frac{\log x}{\log 1-\log 2}=\frac{\log x}{0-\log 2}=-\frac{\log x}{\log 2} = -\log_2 x $ Actually,I was not considering the square root! – 2010-11-20