Question

初二数学题目，约分。

a^2-1/a^2+2a+1

4-m/m^2-16

-x^2+2x-1/x-1

先约分，再求值：
a^2-8a+16/a^2-16,其中a=5

3x^2-xy/9x^2-6xy+y^2,其中x=3/4 y=-2/3

Answer 1

【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

Answer 2

a^2-1/a^2+2a+1
=(a+1)(a-1)/(a+1)^2
=(a-1)/(a+1)
4-m/m^2-16
=-(m-4)/(m+4)(m-4)
=-1/(m+4)
-x^2+2x-1/x-1
=-(x-1)^2/x-1
=1-x
先约分，再求值：a^2-8a+16/a^2-16,其中a=5
a^2-8a+16/a^2-16
=(a-4)^2/(a+4)(a-4)
=(a-4)/(a+4)
=1/9
3x^2-xy/9x^2-6xy+y^2,其中x=3/4 y=-2/3
3x^2-xy/9x^2-6xy+y^2
=x(3x-y)/(3x-y)^2
=x/3x-y
=3/4 /(9/4+2/3)
=9/35

Answer 3

1. a^2-8a+16 =（a-4)的平方
a^2-16=(a+4)(a-4) 分子和分母同时约掉一个（a-4） 是剩下（a-4)÷（a+4) 把a=5带入 所以结果为1/9.
2 第二题直接把数代入就能求出来了

Answer 4

a-1╱a+1
-1╱m-4
a+4╱a-4 9
x╱3x-y 6╱1