Why Does $\Sigma_{i=1}^n x_i(y_i - \alpha - \beta x_1) = 0$ Follow?

[superwolf]

Why does it follow from

$$\frac{d}{d\beta}(-\Sigma_{i=1}^n[\frac{(y_i - \alpha - \beta x_i)^2}{2 \sigma^2}]) = 0$$

that

$$\Sigma_{i=1}^n x_i(y_i - \alpha - \beta x_1) = 0$$

?

 

[diazona]

What happens when you take the derivative?

This one won't be hard ;-)