$\sum (y_i - \bar{y})^2 = \sum (\hat{y}_i - \bar{y})^2 + \sum (y_i - \hat{y}_i)^2$

$\sum (y_i - \hat{y}_i)^2 = \sum (\hat{y}_i - \bar{y})^2 + \sum (y_i - \bar{y})^2$

$SST = SSE + SSR$

$SST = SSR + SSE$