DianeD -

Maybe you need to use more significant digits. Here's the results I get when
I click the Increase Decimal button repeatedly on the trendline formula.

Dataset 1
y = 0.000083433350134*x^2 + 0.002640708447384*x + 0.006131225633048

Dataset 2
y = 0.000003011103155*x^3 - 0.000436225909118*x^2 + 0.027227135337526*x +
0.262411805746881

Or, you could use array-entered LINEST on the worksheet:

Dataset 1
=LINEST(known_y's,known_x's^{1,2})
8.343335013361470E-05 2.640708447383810E-03 6.131225633048880E-03

Dataset 2
=LINEST(known_y's,known_x's^{1,2,3})
3.011103155272290E-06 -4.362259091179450E-04 2.722713533752600E-02
2.624118057468820E-01

- Mike Middleton
http://www.DecisionToolworks.com
Decision Analysis Add-ins for Excel



"DianeD" wrote in message
...
I have 2 similar questions regarding quadratic (2nd order) and cubic (third
order) polynomial curve fits.

Dataset 1 is comprised of the following (x,y) pairs: (5, 0.02305), (8,
0.03235), (12, 0.04655), (16, 0.07065), (20, 0.09195), (24,0.11935), (32,
0.17605) and (40,0.24485). After X-Y scatter, the resulting polynomial
equation results: y= 8E-05x^2 + 0.002X + 0.006. I have a variety of
addtional
Y, for which I must solve for X using the quadratic formula- everything I
try
works on the linear model, which is unacceptable.

Dataset 2 is comprised of the following (x,y) pairs: (0, 0.2795), (1,
0.2947), (2, 0.3113), (5, 0.3697), (10, 0.4756), (20, 0.6772), (60,
0.9729)
and (100, 1.6345). After X-Y scatter, the resulting polynomial equation
results: y= 3E-06x^3 - 0.000x^2 + 0.027X + 0.262. I have a variety of
addtional Y, for which I must solve for X using the cubic formula-
everything
I try works on the linear model, which is unacceptable.