I was referring to this wiki article http://en.wikipedia.org/wiki/Simple_linear_regression where it tried to calculate the confidence interval of the regression parameters
$$ \hat\alpha(intercept) $$ and
$$ \hat\beta(slope) $$
$$ (\alpha) (\beta) \\ $$ are the true population parameters. It then calculates the variance of beta as
$$ \sigma^2/\sum(x_i-x_m)^2\\ $$ where $$\sigma^2$$ is the variance of error term. I didn't get how this variance was calculated. Also I didn't get the rest of the derivation for the confidence interval. Any suggestions?