If $x-y=15$ and $xy=4$, what is the value of $x^2+y^2$?
I notice that the expression $x^2+y^2$ can be rewritten as $(x-y)^2+2xy$. This might be helpful, because I already know the values of $x-y$ and $xy$. So I substitute those values and get $(15)^2+2(4)$. This simplifies to $225+8$, which is $233$.

