# 2nd Order Polynomial Curve-fit using Trend function

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# 2nd Order Polynomial Curve-fit using Trend function

Hello,

I have a small 4-pair (x,y) data set that I'm trying to extrapolate using a 2nd order curve-fit.  I have the  desired value of "y" and I want the corresponding "x", which may be a negative number.  If I plot the data in an Excel chart, and "forecast" the 2nd order trendline, I get a reasonable answer.  If I use the LINEST function, calculate the polynomial coefficients and solve the quadratic equation, I get a reasonable answer if I control the (+/-) option on the quantity +/-SQRT(B^2-4AC).  There's two real solutions (roots) to the quadratic.  Using the -SQRT(B^2-4AC) option, yields a result which is compatible with the trendline forecast on the Excel chart.

However, when I use the TREND function to extrapolate the dataset as a 2nd order polynomial, the result is limited to the value which is produced when solving the quadratic equation with the +SQRT(B^2-4AC) formulation.  Is there anyway for me to control the TREND function to avoid this ambiguity?

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