2b^2-b b-1
---------- - ----------- = ?
b^3+1 b^2-b+1

(2b²-b)/[(b+1)(b²-b+1)]-(b-1)/(b²-b+1)==[(2b²-b-(b+1)(b-1)]/[(b+1)(b²-b+1)]=(2b²-b-b²+1)/[(b+1)(b²-b+1)]=(b²-b+1)/[(b+1)(b²-b+1)]=1/(b+1)