Two surveys were independently conducted to estimate a population mean, . Denote the estimates and their standard errors by and and and . Assume that and are unbiased. For some and , the two estimates can be combined to give a better estimator:

a.Find the conditions on and that make the combined estimate unbiased.

b.What choice of and minimizes the variances, subject to the condition of unbiasedness?

Answer: (a)

Answer: (b) ,

I don't know how to get the answer for part (b)! please help