先化简，再求值： $\frac{\mathit{a}\mathit{-}\mathit{4}}{\mathit{a}}\mathit{÷}\mathit{(}\frac{\mathit{a}\mathit{+}\mathit{2}}{{\mathit{a}}^{\mathit{2}}\mathit{-}\mathit{2}\mathit{a}}\mathit{-}\frac{\mathit{a}\mathit{-}\mathit{1}}{{\mathit{a}}^{\mathit{2}}\mathit{-}\mathit{4}\mathit{a}\mathit{+}\mathit{4}}\mathit{)}$，其中 $\mathit{a}\mathit{=}\sqrt{\mathit{2}}$．