Hi,

If y=ax^2+bx+c, then ax^2+bx+c-y = 0. So for any given y-value
(Absorbance), you can solve for the x-value (protein concentration) by using
the quadratic formula:

x = (-b + sqrt(b^2-4*a*(c-y)))/(2*a) or (-b - sqrt(b^2-4*a*(c-y)))/(2*a)

When you plug in the values of a, b, c, and y, only one of the above
equations would give a meaningful solution for x (the other one will give a
value for x that is physically absurd); my guess is that the first equation
will give the meaningful x-value for any y-value, but you should try.

So if you have 75 absorbance values in a column (say A2:A76), in B2 enter
the following formula (of course enter the actual values of a, b, and c from
the polynomial fit in place of "a", "b" and "c" in theformula)

=(-b+SQRT(b^2-4*a*(c-A2)))/(2*a)

and autofill the formula down to B76. If the values are not satisfactory,
try changing the "+" sign into "-" and see whether it helps).

Regards,
B. R. Ramachandran

"lizzystien" wrote:

Okay, I'm more than likely just very dense with all of this, but I'm still
having troubles. My equation is in the form y=Ax^2+Bx=C. For the graph that
I used to create the line, the y variable indicates absorbancies while the
x-values are mg of protiens. My 75 known values are absorbancies and I need
to predict the amount of protien for each aborbancy. Still haven't had much
luck, but thank you to bpeltzer for at least trying to teach the unteachable.

"lizzystien" wrote:

I have an xy scatter plot constructed and created a polynomial function for
the line. I want to use the equation obtained to calculate the expected x
values from 75 different y values without punching it all into a calculator.
I know there is a way to do this on excel, but it's been awhile, anyone
willing to be a teacher?