I have learnt things I didn't know previously. If the Excel trendline equation delivers inaccurate results when you supply x values to it manually. At the same time I can see how increasing the number of decimal places at least in some cases gives better results. Excel Polynomial Trendline Not Working Properly. So in summary, I am at a loss to understand why the equations in the first worksheet were wrong, yet in a new worksheet using the same table of data the equations were okay. The equation became y = 0.000640x^2 - 0.144000x + 10.400000 and now all 3 original and 4 extra values of x gave very good agreement, as was the case with the first chart. So for the second chart I increased the equation to 6 decimal places. Nor did the other 4 points exactly match the trendline. When I tested the formula (with 2 decimal places) the values didn't match exactly the 3 original values of y. In the case of the second chart the equation had changed from y = 0.68x^2 + 2.05x + 99.75 to y = 0.00x^2 - 0.14x + 10.40.

All this with a setting of 2 decimal places. Find intersection of polynomial interpolated lines. I tried another 4 values of x and all gave y values that appear to match the trendline. Use Trend Line Equations to Get an Intersection Point in Excel 2. I then used the new equation with the 3 original x values and all got exactly the correct y values. Amazingly, at least to me, while the 3 points for each chart were unchanged, the equations changed radically. Since my last post I have created a new worksheet and recreated the 2 charts and then added trendlines and equations.

Excel polynomial trendline equation