Bài 1:
`a,7x^2-14xy+7y^2`
`=7(x^2-2xy+y^2)` `=7(x-y)^2`
`b,xy-9x+y-9` `=(xy+y)-(9x+9)`
`=y(x+1)-9(x+1)` `=(x+1)(y-9)`
Bài 2:
`a,ĐKXĐ: x\ne +-2`
`b, A=((2+x)/(2-x)-(4x^2)/(x^2-4)-(2-x)/(2+x)):(1-2x)/(2-x)` `=((2+x)/(2-x)-(4x^2)/((x-2)(x+2))-(2-x)/(2+x)):(1-2x)/(2-x)` `=[(2+x)^2/((2-x)(2+x))-(4x^2)/((x-2)(x+2))-(2-x)^2/((2+x)(2-x))].(2-x)/(1-2x)` `=(4+2x+x^2-4x^2-4+2x-x^2)/((2-x)(2+x)).(2-x)/(1-2x)`
`=((4x-4x^2)(2-x))/((2-x)(2+x))` `=(4x(x-1))/(2+x)`
`c,` Khi `x=-3/4`, ta có:
`A=(4.(3/4)(3/4+1))/(2-3/4)=7/(5/4)=28/5`
