`ĐKXĐ : x-1` khác` 0 ; x+1 `khác `0;x^2+x+1`khác `0`
`x `khác` ±1`và `x` khác` 1/2``
`A=[1/(x-1) -x/(1-x^3).(x^2+x+1)/(x+1)]:(2x+1)/(x^2+2x+1)`
`⇔A=[1/(x-1) +x/(x^3-1).(x^2+x+1)/(x+1)]:(2x+1)/(x^2+2x+1)`
`⇔A=[1/(x-1) +x/[(x-1)(x^2+x+1)].(x^2+x+1)/(x+1)]:(2x+1)/(x^2+2x+1)`
`⇔A=[1/(x-1) +x/[(x-1).(x+1)]]:(2x+1)/(x^2+2x+1)`
`⇔A=[(x+1+x)/[(x-1).(x+1)]]×(x^2+2x+1)/(2x+1)`
`⇔A=[(2x+1)/[(x-1).(x+1)]]×(x+1)^2/(2x+1)`
`⇔A=(x+1)/(x-1)`
thay `x=1/2`
`⇒A=(1/2 +1)/(1/2-1)=(3/2)/(-1/2)=3/2 ×-2=-3`