Excel Exponent & LN

M

me28

I need to create a worksheet that takes an active value and calculates the
timed decay. The formula is A e-(0.693t/T½) (e to the power of (0.693t,
though LN may work). Value of A = Initial Value, T = Timed decay.
 
D

David Biddulph

Well, the first problem is that your time elapsed is A5-B2, not B2-A5.

Secondly, using your formula (or correcting the sign of t) I don't get
anything remotely like 199.7 or 197.46. I get 1.06E+09 with your formula,
and 4.00E-05 correcting the value of t. I assume that your half-life is in
years, not days, so if you correct for that you'll get your 197.46, or
thereabouts.
 
B

Bernard Liengme

Surely you mean =$B$1*EXP(-(0.693*((A5-B2)/(30*365))))
which gives 197.460
What makes you think the "correct" value is 199.70?
If I replace 0.693 by LN(2) to improve the precision and 365 by 365.25, I
get only 197.464
I think you have been given the wrong answer! Take it from a retired chem.
prof that textbook errors are not rare.

Note that my formula does give a result that decrease as time does on (ie as
A5 increases)
 
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