The monthly demand for a certain brand of toaster is given by a function f (x,y) (this demand function produces a value that is the number of toasters sold). x is the selling price (in dollars) of this toaster and y is the selling price (in dollars) of the main competitor’s toaster. Under normal economic conditions, what will be the sign of each of the following derivatives? You only need to give a final answer; no supporting work is required here.
(a) fx(20,25)
(b) fy(20,25)