Consider a sample of 20 observations of the model Yi = a+BXi +ui, where the errors are such that ui~ N(0,02) e independent and identically distributed. This sample yields the following data: EYI = 21.9; (Yi-Y )2 = 86.9; (Yi-Y)(XI-X) = 106.4; EXi = 186.2; E(Xi -X)2 = 215.4. a) Find the estimators a^ and B^. b) Find the estimated standard deviations for a^ and B. c) Area and B statistically different from zero at the 95% confidence level? With a confidence level of 99%?