计算1=a^3/b^3xy(x+y)/(x-y)^2*(x-y)/(xy+y^2)=x/(x-y)(16-a^2)/(a^2+8a+16)÷(a-4)/(2a+8)=(4+a)(4-a)/(a+4)^2÷(a-4)/2(a+4)=-2(a-1)/(a-2)÷（a^2-2a+1)/(2a-4)=(a-1)/(a-2)*2(a-2)/(a-1)^2=2/(a-1)
