This is a problem I have every once in a while, and it annoys me tremendously, beacuse I have always to recheck every trendline I get.

An example:

```
r L
(mm)
30,00 97,0
60,00 103,2
90,00 106,0
110,00 101,0
125,00 88,0
140,00 62,0
148,00 36,7
152,50 17,0
```

Upon drawing a trendline (using 3rd order polynomial regression type) with *r* on the x axis, and *L* on the y one, Excel will give the formula

y = -0,0002x³ + 0,0341x² - 1,8979x + 128,73

with R² = 0,994. If I interpolate values using that formula for the same values of *r* as the ones the formula was derived from, I get

```
r y
(mm)
30,00 97,083
60,00 94,416
90,00 88,329
110,00 66,371
125,00 33,68
140,00 -17,416
148,00 -53,5912
152,50 -76,97725
```

which are quite different?

*Why does this happen? What is the reason for it?*