dx/dt=-3-3t²
dy/dt=-3+3t²
y'=(dy/dt)/(dx/dt)=(-3+3t²)/(-3-3t²)=(1-t²)/(1+t²)
由y'=0得：t=1,-1
dy'/dt=-4t/(1+t²)²
y"=-4t/(1+t²)²/(-3-3t²)=(4t/3)/(1+t²)³
t=-1时,y"