You have two unbiased estimates, e1 and e2, of a quantity with variances V1 and V2 respectively, and covariance Cov{e1,e2} = C. Form a new unbiased estimate:
e3 = a1e1 + a2e2
Find a1 and a2 to minimize the variance of e3.
1. What is the variance of e3?
2. Specialize to the case V1 = V2 = V and C = cV.