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I have made a standard curve. The X-axis is logarithmic. The y-axis is linear. I have added a logarithmic trendline (y = -1.546ln(x) + 39.254; R² = 0.9906). How can I re-arrange the equation to calculate an unknown X-axis value?

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  • $\begingroup$ *from a known y-value $\endgroup$ – Jessica Oliver-Bell Apr 30 '13 at 13:25
  • $\begingroup$ just copy the equation you got into excel cell and place in it instead of X - put the cell name that contains the X value you want $\endgroup$ – user20965 Jul 10 '16 at 14:14
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$$y = -1.546 \ln(x) + 39.254$$ $$\frac{39.254 - y}{1.546} = \ln(x)$$ $$\exp\left(\frac{39.254 - y}{1.546}\right) = x$$

In excel this is =EXP((39.254 - Y)/1.546) where Y is the cell address containing the Y value.

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