$In$ each part below determine the value $of$ the given $limitor$ show that the $limit$ does not exist.

$(i)\underset{(x,y)(0,0)}{lim}\frac{x^2-4xy+2y^2}{x^2+y^2}$

$(ii)\underset{(x,y)(0,0)}{lim}\frac{x^3{y}^2}{(x^2+y^2{)}^2}$

$(iii)\underset{(x,y)(0,0)}{lim}\frac{x{y}^3}{x^2+y^6}$