讨论函数f(x,y)=y²-x²+1是否有极值令∂f/∂x=-2x=0,得x=0；∂f/∂y=2y=0,得y=0；故得唯一驻点(0,0)；在点(0,0)求得：A= ∂²f/∂x²=-2；B=∂²f/∂...