I originally tried LOGEST() but couldn’t properly relate the CALC documentation to my spreadsheet content. However I had another look following your comment and, interestingly, it doesn’t show the same ERR:502 problem, at least for ‘a’.
If y = ae^bx I can identify the correct value for the exponent ‘b’ in the array returned by LOGEST() and a plausible value for RSQ(), but I cannot see how to calculate ‘a’. And this assumes the exponential function should be recast as
ln(y) = ln(a) + bx*ln(e) or ln(y) = ln(a) + bx
The values for ‘a’ & ‘b’ returned by the formulae I actually used agree exactly with those returned by the exponential-trend option of the chart. SLOPE() is necessary for calculating ‘b’ but not ‘a’ and I think I only quoted the former.
Thanks for your interest…
DavidL