初一数学问题28.1/a-1+1/a+1-2/a^2-129.3-x/2-x÷（x+2-5/x-2）30.x（y-x）÷x^2-2xy+y^2/xy*x-y/x^231.先化简,再求值：1/x+1-x+3/x^2-1*x^2-2x+1/x^2+4x+3,其中x=2.32.已知1/x-1/y=3,求5x+xy-5y/x

Question

初一数学问题
28.1/a-1 +1/a+1 -2/a^2-1
29.3-x/2-x ÷ （x+2- 5/x-2）
30. x（y-x）÷ x^2 - 2xy+y^2/xy * x-y/x^2
31.先化简,再求值：1/x+1 - x+3/x^2 - 1 * x^2 -2x+1/x^2 +4x+3,其中x=2.
32.已知1/x -1/y =3,求 5x+xy -5y/x-xy-y的值.

Answer 1

分式的分子、分母要加括号 “ ”表示啊!
28.1/(a-1) +1/(a+1 ) -2/(a^2-1)
=(a+1+a-1-2)/(a^2-1)
=(2a-2)/(a^2-1)
=2/(a+1)
29.(3-x)/(2-x ) ÷ [(x+2)- 5/(x-2）]
=(3-x)/(2-x)/[x^2-4+5]/(2-x)
=(3-x)/(2-x)*(2-x)/(x^2+1)
=(3-x)/(x^2+1)
30.x（y-x）÷ (x^2 - 2xy+y^2)/xy * (x-y)/x^2
=x(y-x)*xy/(x-y)^2*(x-y)/x^2
=-y
31.先化简,再求值：1/x+1 - x+3/x^2 - 1 * x^2 -2x+1/x^2 +4x+3,其中x=2.
1/（x+1） - （x+3）/（x^2 - 1 ） * （x^2 -2x+1）/（x^2 +4x+3）
=1/(x+1)-(x+3)/(x^2-1)*(x-1)^2/[x+1)(x+3)]
=1/(x+1)-1/(x+1)
=0
32.已知1/x -1/y =3,求 5x+xy -5y/x-xy-y的值
由已知可得：1/x-1/y=3
y-x=3xy
则：（5x+xy -5y）/（x-xy-y）
=[5（x-y)+xy]/(3xy-xy)
=16xy/2xy
=8 .