y=ln(x^2-1) y ' = (2x)/(x2 - 1) y'' = [2(x2-1)-2x*2x]/(x2-1)2 =2(1-x2)/(x2-1)2 =2/(1-x2) 用到的公式： y = lnx ， y' = 1/x y=x2 ，y' = 2x y = u(x)/v(x) y' =[ u'(x) * v(x) - u(x) * v'(x)] / [v(x)]2
