Q$2.U_{\text{D}\text{F}\text{A}}{U}_{\text{D}\text{F}\text{A}}=\{is$ a Deterministic Finite Automaton $(DFA)$ and $it$ has $at$ least one useless state"$\}.("That$ means there $isat$ least one state such that for any input string this state cannot $be$ reached.$)$

Prove that $U_{\text{D}\text{F}\text{A}}is$ a decidable language.