polynomial equation

P

PAUL GRAZIDE

I am setting up a calibration curve, where x= concentration and y=
absorbance. The curve is polynomial.

after plotting x and y values using a scattergraph and applying a trend
line, i got the following regression line:
y= -0.0881x^2+2.8089x+0.0505.

If the absorbance reading (y) = 0.212, how do I calculate the value of x?

Thanks in advance
 
D

davesexcel

0.057672821
y= -0.0881x^2+2.8089x+0.0505.

If the absorbance reading (y) = 0.212, how do I calculate the value of
x?
I am just thinking
A1=B2
B2=-.0881*C1^2+2.8089*C1+.0505
then I went to Tools, Goal seek
set the value of A1 to .212 by changing Cell C1
 
G

Gary''s Student

You are correct davesexcel:

This quadratic has two separate solutions:
one around .0576 and the other around 31.82548752

=(-2.8089+(2.8089^2-4*(-0.0881)*(-0.1615))^0.5)/(2*(-0.0881))
=(-2.8089- (2.8089^2-4*(-0.0881)*(-0.1615))^0.5)/(2*(-0.0881))
 
J

Jerry W. Lewis

Or if you do use the chart equation, format it to display scientific notation
with 14 decimal places to avoid rounding issues.

More fundamentally, are you sure that a 4- or 5-parameter logistic would not
be a better model for your data than a polynomial?

Jerry
 
Top