The set of real values of x for which $(\log_x 2) (\log_{2x} 2) (\log_2 4x) >1$, is |
$(1,2^{\sqrt{2}})$ $(1/\sqrt{2},1)$ $(2^{-\sqrt{2}},1/2)$ $(2^{-\sqrt{2}},1/2)∪(1,2^{\sqrt{2}})$ |
$(2^{-\sqrt{2}},1/2)∪(1,2^{\sqrt{2}})$ |
Various terms in the given inequation are meaningful if $x > 0$ and $x ≠ 1$. Now, $(\log_x 2) (\log_{2x} 2) (\log_2 4x) >1$ $⇒\frac{\log_2 4x}{\log_2 x.\log_2 2x}>1$ $⇒\frac{(2+\log_2 x)}{\log_2 x(1+\log_2 x)}-1>1$ $⇒\frac{(\log_2 x+2)-\log_2 x(1+\log_2 x)}{\log_2 x(1+\log_2 x)}>1$ $⇒\frac{(\log_2 x)^2-2}{\log_2 x(\log_2 x+1)}<0$ $⇒\frac{(\log_2 x+\sqrt{2})(\log_2 x-\sqrt{2})}{\log_2 x(\log_2 x+1)}<0$ The signs of the expression $\frac{(\log_2 x+\sqrt{2})(\log_2 x-\sqrt{2})}{\log_2 x(\log_2 x+1)}$ for different values of $\log_2 x$ are shown in Fig. $∴\frac{(\log_2 x+\sqrt{2})(\log_2 x-\sqrt{2})}{\log_2 x(\log_2 x+1)}<0$ $⇒-\sqrt{2}<\log_2x<-1$ or, $0<\log_2x<\sqrt{2}$ $⇒2^{-\sqrt{2}}<x<\frac{1}{2}$ or, $1<x<2^{\sqrt{2}}$ $⇒x∈(2^{-\sqrt{2}},1/2)∪(1,2^{\sqrt{2}})$ |