【参考答案】
1、(a^2-1)/(a^2+2a+1)
=(a+1)(a-1)/(a+1)²
=(a-1)/(a+1)
2、(4-m)/(m^2-16)
=(4-m)/[(m+4)(m-4)]
=-1/(m+4)
3、(-x^2+2x-1)/(x-1)
=-(x-1)²/(x-1)
=-(x-1)
=1-x
先约分,再求值：
1、a^2-8a+16/a^2-16,其中a=5
原式=(a-4)²/[(a+4)(a-4)]
=(a-4)/(a+4)
=1/9
2、3x^2-xy/9x^2-6xy+y^2,其中x=3/4 y=-2/3
原式=x(3x-y)/(3x-y)²
=x/(3x-y)
=(3/4)/[(9/4)+(2/3)]
=9/(27+8)
=9/35