i need to know the reverse of a log10 funtion to get back to the number.

If A1 contains the result of your LOG10 formula (e.g., =LOG10(1.234)),

then the antilog is =10^A1.

Although that does return exactly 1.234 in that case, in general do

not expect the antilog to exactly equal the parameter of LOG10. For

example, if A1 is =LOG10(PI()), =10^A1-PI()=0 is FALSE(!) [1].

Also, do not expect the antilog to exactly match mathematical

equalities. For example, if A1 is =LOG10(4.5)+LOG10(2) [2], =10^A1=9

is FALSE(!).

Infinitesimal differences are due to the limitations of computer

arithmetic as well as to the fact that generally LOG10 and the power

operator (^) use generating functions or algorithms to approximate

their results (when the exponent is non-integer in the case of the

power operator).

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Endnotes

[1] But =10^A1=PI() is TRUE. The difference is due to Excel

heuristics which try to hide inequalities when the difference is

"close" to zero.

[2] We expect LOG10(4.5)+LOG10(2) = LOG10(4.5*2) = LOG10(9) based on

mathematical equalities.