$35-54.$ Continuity At what points of $R^2$ are the following functions continuous?

$35.f(x,y)=x^{2}2xy-y^3$

$36.f(x,y)=\frac{xy}{x^2{y}^{2}1}$

$37.p(x,y)=\frac{4x^2{y}^2}{x^4{y}^2}$

$38.S(x,y)=\frac{2xy}{x^2-y^2}$

$39.f(x,y)=\frac{2}{x(y^{2}1)}$