(a) Let X1, ..., Xn be a sample of real valued data, and let Xn = _ __, X; denote the sample mean. Show that n Xn = arg min E(x, - a)? aER j=1 (b) Now consider a sample X1, ..., Xn of d-dimensional feature vectors, d > 2, and let Xn be the sample mean (also a d-dimensional vector). Show that n Xn = arg min aERd j=1