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$\mbox{ Find }x \in \mathbb{Q} \mbox{ to keep the equality: } \sqrt[2x+1]{\sqrt[11-4x]{(-2x)^{3x}}}=\sqrt[3x-1]{7x+2}$

I tried to write the roots using powers:

\begin{align*}\sqrt[2x+1]{\sqrt[11-4x]{(-2x)^{3x}}}=\sqrt[3x-1]{7x+2}&\Rightarrow [(-2x)^{\frac{3x}{11-4x}}]^{\frac{1}{2x+1}}=(7x+2)^{\frac{1}{3x-1}}\\ &\Rightarrow (-2x)^{\frac{\frac{3x}{11-4x}}{2x+1}}=(7x+2)^{\frac{1}{3x-1}}\\ &\Rightarrow (-2x)^{\frac{3x}{(11-4x)(2x+1)}}=(7x+2)^{\frac{1}{3x-1}}\\ &\Rightarrow (-2x)^{\frac{3x}{-8x^2+18x+11}}=(7x+2)^{\frac{1}{3x-1}} \end{align*}

I hope I did it right until this point. But I've stuck here. Can someone help me?
Thanks.

  • 0
    Is $3x-1$ correct? I don't see how could there exist three positive integers $2x+1$, $11-4x$ and $3x-1$, with -2/7. ([nth root](http://en.wikipedia.org/wiki/Nth_root) )2011-10-04

2 Answers 2

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Take logs and then use your favourite numerical root-finding algorithm. Hint: the logarithms are only both real for $-\frac{2}{7} < x < 0$.

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I agree with Peter Taylor.

You get:

\begin{equation} \frac{3x}{(2x+1)(11-4x)}\ln(-2x) = \frac{1}{3x-1}\ln(7x+2) \end{equation}

Graphing gives about -0.15649: http://bit.ly/q5Rn4l

I don't think you can solve it exactly. A rational solution is possible but unlikely in general. Maybe you're meant to just try a whole bunch of rational numbers...