a的平方-4/a的平方+2a-8a除以（a的平方-4）乘以a的平方-2a/a+2
=a(a+2)/(a+2)(a-2)-[8a/(a+2)(a-2)]*(a-2)/a(a-2)
=a/(a-2)-8/(a+2)(a-2)
=(a^2+2a-8)/(a+2)(a-2)
=(a+4)(a-2)/(a+2)(a-2)
=(a+4)/(a+2)