By considering different paths of approach, show that the following functions have no limit as (x,y) (0,0): (a) f(x, y)=- (b) f(x,y)= X x + y (c) f(x, y) = (d) f(x, y) = x-y x + y (c) f(x, y) = xy |xy| x + y At what points (x, y) in the plane are the following functions continuous? x + y (a) f(x,y)=sin(x + y) x-y