How is this formula derived
slope= $\displaystyle \frac{\sum(x_{i}-x_{0})(y_{i}-y_{0})}{\sum(x_{i}-x_{0})^2}$
This seems to give me the average ratio of the slope
$\displaystyle \frac{\sum(y_{i}-y_{0})}{\sum(x_{i}-x_{0})}$
I know this is one, but why is it needed?
$\displaystyle \frac{\sum(x_{i}-x_{0})}{\sum(x_{i}-x_{0})}$