先化简,再求值(a-a/a+1)除a方-2a/a方-4乘1/a+2 其中a=2写详细一点,不要简写= =

原式=[a-a/(a+1)]÷[(a²-2a)/(a²-4)]×[1/(a+2)]
=[a(a+1)/(a+1)-a/(a+1)]÷{a(a-2)/[(a+2)(a-2)]}×[1/(a+2)]
=[(a²+a-a)/(a+1)]÷[a/(a+2)]×[1/(a+2)]
=[a²/(a+1)]×[(a+2)/a]×[1/(a+2)]
=a/(a+1)
=2/(2+1)
=2/3

原式=1-a+1/a-2/a-4/a+2
=3-a-5/a
代入 a=2
得 3-2-5/2=-1.5

原式=[a-a/(a+1)]÷[(a²-2a)/(a²-4)]×[1/(a+2)]
=[a(a+1)/(a+1)-a/(a+1)]÷{a(a-2)/[(a+2)(a-2)]}×[1/(a+2)]
=[(a²+a-a)/(a+1)]÷[a/(a+2)]×[1/(a+2)]
=[a²/(a+1)]×[(a+2)/a]×[1/(a+2)]
=a/(a+1)
=2/(2+1)
=2/3