G7.2 Refer to the diagram, $P$ is any point inside the square $O A B C$ and $b$ is the minimum value of $P O+P A+P B+P C$, find $b$.
$\begin{array}{l} P O+P A+P B+P C \geq O B+A C \text { (triangle inequality) } \\ =2 O B \\ =2 \sqrt{1^{2}+1^{2}} \\ \Rightarrow b=2 \sqrt{2}\end{array}$