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# Exponential trendline fit in Excel using LINEST - fixing a variable

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 GrahamSRowe (Bioengineer) 31 Dec 10 3:10
 I know that you can use the following formulae in Excel to fit a curve using LINEST for the named ranges x and y:Exponential TrendlineEquation: y = c *e ^(b * x)c: =EXP(INDEX(LINEST(LN(y),x),1,2))b: =INDEX(LINEST(LN(y),x),1)My question is, if I want to fix c=1 can I simply useb: =INDEX(LINEST(LN(y),x),1) and assume c=1?Also would it be correct to user2: = INDEX(LINEST(LN(y),x),TRUE,TRUE),3) in this case?Thanks very much.
 Brad1979 (Electrical) 10 Jan 11 15:36
 The syntax for LINEST is:LINEST(known_y's,known_x's,const,stats)You can force the intercept to be 0 (which is equivalent to c=1) by setting const to FALSE.  So your equations should be:c: =EXP(INDEX(LINEST(LN(y),x, FALSE),1,2))b: =INDEX(LINEST(LN(y),x, FALSE),1)This will cause b to be calculated assuming that c = 1.
 Brad1979 (Electrical) 10 Jan 11 15:47
 Also, r2: = INDEX(LINEST(LN(y),x),TRUE,TRUE),3) this is correct in general but you'll want to change it to the following when you for c=1:r2: = INDEX(LINEST(LN(y),x),false,TRUE),3)
 Denial (Structural) 10 Jan 11 16:24
 Note typo in Brad1979's last two formulae where "x)," should be "x,"
 Brad1979 (Electrical) 10 Jan 11 16:41
 Yes, thank you for catching that.
 BigInch (Petroleum) 17 Jan 11 8:33

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