I'd like to use these data to derive an equation using Excel.

```
300 13
310 12.6
320 12.2
330 11.8
340 11.4
350 11
360 10.8
370 10.6
380 10.4
```

As x goes up, y goes down. Seems straightforward. But when I do a polynomial regression on these data, even though the trendline matches the data pretty well, the equation it generates doesn't work. The equation is `0.0096x2 - 0.4181x + 13.341`

When I plug in x values to that equation, the numbers go up! So something is pretty wrong here.

My steps:

- place both number series in excel
- select the second set (13, 12.6 ...)
- plot a line graph
- set the first set as the x axis labels
- select Series1 and add a polynomial (2) trendline, display equation, display R-squared

That produces the equation above, with an R^2 value of .9955. But when I use that equation, it doesn't produce those outputs for those inputs.

Clearly I'm doing something wrong.

Edit: or is it excel? Here's the graph of that equation (above):

Clearly that **is not trending downward for the range 300-390**.

Here's the real equation that fits this data:

Quadratic Fit: y=a+bx+cx^2
Coefficient Data:

a = 4.53E+01
b = -1.66E-01
c = 1.95E-04

Thank you CurveExpert 1.4.