[（a^2+2a分之a-2）-（a^2+4a+4分之a-1）]除以a+2分之a-4,其中a=-3 =[(a-2)/a(a+2)-(a-1)/(a+2)²]÷(a-4)/(a+2)=[(a²-4-a²+a)/a(a+2)²]*(a+2)/(a-4)=[(a-4)/a(a+2)²]*(a+2)/(a-4)=1/a(a+2)...