Originally Posted by

**BobP** The equation that you should be differentiating is the one for $\displaystyle S.$

Differentiating wrt $\displaystyle a$ gives you

$\displaystyle \frac{\partial S}{\partial a}=-2\sum_{i=1}^{n}(y_{i}-a-bx_{i}-cx^{2}_{i})$

and on putting this equal to zero this can be rewritten as

$\displaystyle \sum_{i=1}^{n} y_{i}=an+b\sum_{i=1}^{n} x_{i}+c\sum_{i=1}^{n} x_{i}^{2}$

The summations are taken over the $\displaystyle n$ data points and will therefore be numbers.

Differentiate to find the other two equations and then solve them simultaneously for $\displaystyle a,b$ and $\displaystyle c.$

BTW,** do not double post**. The response that you are getting from your other posting is an alternative method, they produce the same end result.