Var(estimated beta1) = sigma^2/Sxx ****** Cov(estimated beta1, estimated beta0) = -sigma^2*xbar/Sxx ******* Cov(Ybar, b1') = E(Ybar*b1') - E(Ybar)*E(b1') =E(xbar*b1' + b0')*b1' -(xbar*b1+b0)*b1 =xbar*E(b1'^2) +E(b1'*b0') -xbar*b1^2-b0*b1 =xbar(Var(b1')+b1^2) + b1*b0 + Cov(b1', b0') -xbar*b1^2 - b0*b1 = ... =0
