At the same time I can see how increasing the number of decimal places at least in some cases gives better results. 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. Im trying to determine the percentage rate of change in a trendline. You can click the titles of each category list to expand and collapse the options in that category. This type is useful if the input values change with increasing speed. When I tested the formula (with 2 decimal places) the values didn't match exactly the 3 original values of y. To work around this behavior, increase the digits in the trendline equation by increasing the number of decimal places that are displayed. 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. I tried another 4 values of x and all gave y values that appear to match the trendline. I then used the new equation with the 3 original x values and all got exactly the correct y values. Note In previous versions of Excel, you change the number of decimal places in a trendline equation by using the Increase Decimal or Decrease Decimal. 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.
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