y'=(（x-1/x+1)³ )'
=3（x-1/x+1)^2 * (x-1/x+1)'
=3（x-1/x+1)^2 *(1-2/(x+1))'
=3（x-1/x+1)^2 *2/(x+1)^2
=6(x-1)^2/(x+1)^4