4. Let A ( Rox" be symmetric. Let f(x) = 4x Ax -x b. Consider the optimization problem: 1 min -2 Ar - x b CERn 2 (a) Calculate Vf(x) and V- f(x). (b) Assume that A is also positive definite, i.e., I Ax > 0 for all x * 0. Let To be the solution of Ac, = b. Prove that f(x) >f(Io), Vx #, i.e., To is the global minimizer of f