Let f(x,y) = {xy/(x^2 +y^2) , (x,y) not equal to (0,0) , 0 , (x,y) =(0,0)}
(I) show that f(x,y) and f(x,0) are each continuous functions of one variable.
(ii) is f continuous at (0,0)? Give reason for your answer.
(iii) is f differentiable at (0,0)? Give reason.