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Suppose that X1t, X2t and yt are three centered variables with respect to their sample mean. You regressed y, on X and Xze, and you obtained the OLS estimates and the variance covariance matrix of Bis: V=2121 You have n = 52 observations and we assume the errors are normal conditional on the observed regressors 1. Indicate whether each coefficient is significantly different from or not at the level a = 5%. 2. Test the null hypothesis that the sum of the coefficients is zero. 3. Test the joint null hypothesis that the two coefficients are zero