Sxx: Variance Formula
[ \fracS_xx\sigma_x^2 \sim \chi^2_(n-1) ]
[ S_xx = \sum_i=1^n (x_i - \barx)^2 = (n-1) s_x^2 ] sxx variance formula
[ \fracS_xx\sigma_x^2 \sim \chi^2_(n-1) ]
[ S_xx = \sum_i=1^n (x_i - \barx)^2 = (n-1) s_x^2 ] sxx variance formula
