There are 2 objects with weights a and b. We weighed with errors the first, second and both objects together (obtaining

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There are 2 objects with weights a and b. We weighed with errors
the first, second and both objects together (obtaining observations
X1, X2 and X3), and the variance of the error in the last case was
4 times larger. Reduce the problem to a linear regression model and
find least-squares estimates for aa and b.b. Let X1 = 5, X2 = 7, X3
= 18. In your answer, write down the estimate of a.