Breaking down the polynomial

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Hello!

I am using INDEX() &LINEST() to get the coefficients for a quadratic polynomial regression (using =INDEX(LINEST(y:y,x:x^{1,2}),1,3) kind of formulas). I would like to understand every step of the calculation. Does anyone know how to break down the calculation, using SLOPE() and INTERCEPT() ?

Many thanks,

 

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PGC01,
Thank you very much!
Everything is clearer now and am able to understand everything better.

 

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Hello again.

Thanks to your help I managed to understand better all steps of the polynomial. I have it plotted and it matches the trendline provided with excel.

Now I am trying to forward it (forecast), by dragging down the calculations (y = (c2 * x^2) + (c1 * x ^1) + b), but it does not match the forward polynomial of excel

do you have any idea why? i can send you the file if it helps.