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Doing an assignment for maths, I applied a sixth order polynomial trend line to a set of data points I measured and the result was the forming of an equation which seemed legitimate, the trend line visually appearing to fit the data points almost exactly with the r^2 value enforcing this (0.9997). As apart of this assignment I had to derive the function of the trend line and solve for x when the derived function was equal to zero (find stationary point). I did this and using a numerical analysis method found that this point was at 6.2347. Looking at the graph it was obvious that this was not the case, the stationary point appearing to be approximately where x = 4. I checked over this figure and my working and couldn't find any problem so I decided to graph the equation of the trend line. This produced some surprising results with the trend line following the points till the x=3 and then splitting from the data points rather dramatically (see picture). I don't understand why this occurred... In the attached picture it's clear to see that the trend line for the data points series named "Actual Points" is nothing like that of the second series ("Trendline") however Excel still reports the equations of the two lines as being identical.
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So my question is why is it that the two equations in the picture below are stated by Excel as being the same yet obviously are not ?
Thanks
Z.C.
P.S. It should also be noted that I tried this with all other polynomials below 6th order and all of these worked normally.