I need to find $\displaystyle (in R^2) \rho((4,2), A_2)$ where $\displaystyle A_2=\{(x,y)|x^2+y^2=1 \} $.

I know that the definition says that this distance is the greatest lower bound of $\displaystyle \{\rho(x,a)|a \in A\}$. So the distance between $\displaystyle \rho((4,2), (x,y))= \sqrt{(x-4)^2+(y-2)^2}=\sqrt{x^2+8x+16+y^2-4y+4}= \sqrt{21-8x-4y}$ and then I am not sure what to do.