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Here is the original formula: $\frac{256}{2^x}=y$ In order to solve for $x$, I've done this: $\log_{2}\left(\frac{256}{y}\right)=x$ The problem is that $y$ can be zero. What should I do to solve for $x$?

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    $y$ cannot equal zero, because $\frac{256}{2^x}$ can never be equal to $0$.2011-02-26

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If you rewrite $\dfrac{256}{2^x} = y$ as $256 = y \times 2^x$, then it is clearly impossible to have $y=0$ while remaining within the real numbers, as 256 is not a multiple of 0. Since $2^x$ is positive for all real $x$, you must have $y>0$.

So $x = \log_2 \left( \dfrac{256}{y} \right) = 8 - \log_2 \left( y \right)$ is fine.

On the other hand, it is possible to have $x=0$, in which case $2^x = 1$ and so $y = 256$ and $\log_2 \left( y \right) = 8$.

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    I think that is a different question2011-02-27
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HINT $\ $ Rewrite it as $\rm\ Y = 2^{\:8 - X}$