Originally Posted by

**statgirl11**
I'm still very confused about how we can express the term –2*[b1 – Beta1]*Sum[X_i – Xbar]*(u_i – ubar) as –2*((Sum [ X_i – Xbar]*u_i )/ (Sum[ X_i – Xbar]^2)) * Sum[X_i – Xbar]*u_i

How can you just remove the b1 and Beta1 terms? Also, where did the ubar go from the first term? (when I try and manipulate the first expression to get the second, I seem to end up with a ui and ubar still...), also, how did we end up with a (X_i-Xbar) term in the denominator...

I understand why the term –2*((Sum [ X_i – Xbar]*u_i )/ (Sum[ X_i – Xbar]^2)) * Sum[X_i – Xbar]*u_i results in -2*Sigma^2, I just can't seem to get the expression into that form...