先化简,再求值

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+...