1、x²-4x/x²-8x+16
=x(x-4)/(x-4)²
=x/(x-4)
代入
5/（5-4）=5
2、[a/(a-b)-a²/(a²-2ab+b²)]÷(a/(a+b)-a²/(a²-b²)
=[a/(a-b)-a²/(a-b)²]÷[a(a-b)-a²]/(a²-b²)
=[a(a-b)-a²]/(a-b)²÷[a(a-b)-a²]/(a²-b²)
=[a(a-b)-a²]/(a-b)²×(a²-b²)/[a(a-b)-a²]
=(a²-b²)/(a-b)²
=(a+b)/(a-b)
代入
（2+1）/（2-1）=3
为便于区分,加入括号