4. Let A ( RX" be symmetric. Let f(x) = $x Ax - x b. Consider the optimization problem: 1 min - Ar - x b CR 2 (a) Calculate Vf(x) and V'f(x). (b) Assume that A is also positive definite, i.e., x Ax > 0 for all x / 0. Let To be the solution of Aro = b. Prove that f(x) > f(co), Vi / To, i.e., To is the global minimizer of f