How to minimize the following DFA?
States 2 and 5 are distinguishable. We have $(2, 5)\stackrel{b}{\longrightarrow}(1,5)$ and state 1 is a non-final state while state 5 is final. In addition, as HueHang and Amy have noted, $(6, 3)$ is also distinguishable, so the original DFA is already mimimized.