4. Let A be m n and B be m x 1 . Define f : IR"-> R by (a) Quote a previous problem to show that f has a minimum. Say that the minimum (b) Find Df. (Hint: Chain Rule using the function N from Problem 67.) occurs at y E R". (Note: it may be that A. y might be inconsistent.) B, since the equation A. X B (c) Apply the Interior Extreme Theorem to get an equation that must be satisfied by y. (Note: if you get an equation in Ут, take the transpose of the equation to get an equation in y. 4. Let A be m n and B be m x 1 . Define f : IR"-> R by (a) Quote a previous problem to show that f has a minimum. Say that the minimum (b) Find Df. (Hint: Chain Rule using the function N from Problem 67.) occurs at y E R". (Note: it may be that A. y might be inconsistent.) B, since the equation A. X B (c) Apply the Interior Extreme Theorem to get an equation that must be satisfied by y. (Note: if you get an equation in Ут, take the transpose of the equation to get an equation in y.