y'=1/(1-x²)*(1-x²)'=-2x/(1-x²)
y''=-[(2x)'(1-x²)-(2x)(1-x²)']/(1-x²)²
=-[2(1-x²)-(2x)(-2x)]/(1-x²)²
=-(2-2x²+4x²)/(1-x²)²
=-(2x²+2)/(x²-1)²