I have created a chart using Microsoft Excel. I added a polynomial trend line to the created chart, and derived a nonlinear equation for polynomial trend line as y = 0.0008x6 + 0.0013x5 - 0.1054x4 + 0.6517x3 - 1.493x2 + 0.9054x + 0.8343 with R² = 0.9998. I solved the equation by excel by substituting all values of X (From 0.5, 0.75,1, 1.25, 1.5 to 4) into an equation to have the result of Y for each value of X.

The problem is that for the last four values of X from 3.25, 3.5, 3.75 & 4 (in the last part of table) are given wrong result of Y or even in minus values.

I have attached the Excel sheet for more details and help please.

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