Show that the following maping f are linear: f : R? R? defined by f(r, y) = (x+ y, x). f: R3 R defined by f(r, y, z) = 2x 3y + 4z. f: R? R defined by f(r, y) = xy. f: R? R* defined by f(x, y) = (x+ 1, 2y, x+ y). f : R R? defined by f(x,y, z) = (|x], 0).