Evaluate $x+y=xy=3$ how would you evaluate $x^4+y^4$? I know how to evaluate $x^3 +y^3$ when $x+y=xy=3$, but how would you evaluate for? 

$$x^4+y^4$$

Any help would be appreciated.Thanks in advance!

$$(x+y)^2=3^2$$
$$x^2+2xy+y^2=9$$
$$x^2+y^2=9-2xy$$
square both sides
$$x^4+2x^2y^2+y^4=(9-2xy)^2$$
$$x^4+y^4=(9-2xy)^2-2x^2y^2=\color{red}{-9}$$
