3. a) There are many projection matrices in regression theory. Please list three projection matrices which have appeared in regression theory so far and give the statistics where they appear. b) Consider the linear model it = X5 +5, 5 ~ Nn(0,021n), where X : n x p and ,6 = (131, - - - , A319)" is the regression ooeicient vector. What is the hat matrix in the least squares estimator for this model? Give three statistics including the hat matrix in regression analysis and express the related statistical meanings for each statistic