`C = ( 1/(x^2 + 1) - (x + 1)/(x^4 -1)) : (x+ 1)/ (x^5 + x^4 -x -1)`
` = ((x^2 - 1)/(x^4 -1) - (x+1)/(x^4-1)) : (x+1)/(x^4 . (x+1) - (x+1)`
` = (x^2 - 1 - (x+1))/(x^4 -1) : (x+1)/((x^4 -1).(x+1))`
` = (x^2 - 1 - x - 1)/(x^4 - 1) : 1/(x^4 - 1)`
` = (x^2 - x - 2)/(x^4 - 1) . (x^4 - 1)`
` = x^2 - x - 2`
Vậy `C = x^2 - x - 2`