y`_(t)=(t-t^3)`_(t)=1-3t^2
y`_(x)=y`_(t)/x`_(t)=(1-3t^2)/(-2t)=(3t^2-1)/2t
y``_(xx)=(y`_(x))`_(t)/(x`_(t))
(y`_(x))`_(t)=(3t^2-1)/2t=(3/2)t`-(1/2)*(1/t)`=(3/2)+(1/t^2)=(3t^2+2)/(2t^2)
y``_(xx)=(y`_(x))`_(t)/(x`_(t))=((3t^2+2)/(2t^2))/(-2t)=(-1/4)*(3t^2+2)/(t^3)