先化简,再求值:[(a的平方+2a分之a-2)-(a的平方+4a+4分之a-1)除以(a+2分之a-4),其中a=1

[(a-2)/(a²+2a)-(a-1)/(a²+4a+4)]÷(a-4)/(a+2)=(a-2)/(a(a+2))*(a+2)/(a-4)-(a-1)/(a+2)²*(a+2)/(a-4)=(a-2)/(a(a-4))-(a-1)/((a+2)(a-4))=(1-2)/(1*(1-4))-(1-1)/((1+2)(1-4))=-1/(-3)-0=1/3第三步没看明白。。。求解释一下(a-2)/(a(a+2))×(a+2)/(a-4)=(a-2)(a+2)/(a(a+2)(a-4))=(a-2)/(a(a-4))(a-1)/(a+2)²*(a+2)/(a-4)=(a-1)(a+2)/((a+2)²(a-4))=(a-1)/((a+2)(a-4))