((b+1)/4*(b-1)+2/4(b^2-1)-(3+b)/4(b+1))*2(b-1)/3=
=1/(6*(b^2-1))*[b^2+1+2b+2-3b+b-b^2+3]=4/(6*(b^2-1))=2/3(b^2-1)
