`a)A=[2/(x+1)^3 (1/x+1)+1/(x^2 +2x+1)(1/x^2+1)]:(x-1)/x^4`
`=[2/(x+1)^3 .(x+1)/x +1/(x+1)^2 . (1+x^2)/x^2 ]:(x-1)/x^4`
`=[2/(x(x+1)^2)+(1+x^2)/(x^2 (x+1)^2)]:(x-1)/x^4`
`=(2x+1+x^2)/(x(x+1)^2) . x^4/(x-1)`
`=(x+1)^2/(x+1)^2 . x^3/(x-1)`
`=x^3/(x-1)`
`b)A<0`
`<=>x^3/(x-1)<0`
`<=>[({(x^3 >0),(x-1<0):}),({(x^3 <0),(x-1>0):}):}`
`<=>[({(x>0),(x<1):}(n)),({(x<0),(x>1):}(l)):}`
`<=>0<x<1`
Vậy `A<0<=>0<x<1`
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