（a²-2a分之a+2减去a²-4a+4分之a-1）÷a²-2a分之4-a
=( (a+2)/(a²-2a) - (a-1)/(a-2)² ) ÷ (4-a)/(a²-2a)
=( (a+2)(a-2)/a(a-2)² - (a-1)a/(a-2)²a ) × a(a-2)/(4-a)
= (a-4)/a(a-2)² × a(a-2)/(4-a)
=-1/(a-2)
=1/(2-a)