Logarithmic addition and subtraction

Dec 2014
9
1
United Kingdom
Hi

I've got two semilog functions, y1 and y2. I want to be able to recreate y2 from y1. I've found that log(y2)-log(y1) gives me data that are fitted by a second order polynomial.

\(\displaystyle diff = log(y2)-log(y1).\)

I want to be able to add this second order polynomial to y1 so that I can get y2. (I've got a bunch of experiment data, all of which has y1 data but not always y2 data - my aim is to be able to create the y2 data for any of the experiments knowing just y1 and the 2nd order polynomial).

I've tried various combinations of log(y1)+log(diff) but these obviously don't work. Can anyone point me in the right direction?

Thanks
lemmy
 

Debsta

MHF Helper
Oct 2009
1,346
623
Brisbane
log(y2) = log(y1) + diff .... there's no need for a log of diff here

y2 = 10^(log(y1) + diff) if log means base 10, otherwise e^(log(y1) + diff)
 
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Dec 2014
9
1
United Kingdom
Hi

Thanks for the reply. It works! Much appreciated.

Kind regards
lemmy
 
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