Derive the least squares estimates for Beta_1 and Beta_0 shown 
in the Chapter 5 notes (the "computational formula" 
for Beta_1). 

Start with the SSE and take derivatives with respect to Beta_0 
and with respect to Beta_1 (similar to what we did in class for 
the simpler no-intercept model). Setting each of these equal to 
zero gives you 2 equations with 2 unknowns. (Note: the Beta_0 
result can be derived by just considering the derivative with 
respect to Beta_0. The Beta_1 result requires information
from both.)

Show your steps in the derivation.