求Z=x^3-y^3-3xy的极值和极值点

Z=f(x,y)=x^3-y^3-3xy分别对x,y求偏导:fx=3x^2-3yfy=-3y^2-3x.令fx=0,fy=0,可得x=0,y=0,或x=-1,y=1这2个驻点.然后求二阶偏导:fxx=6x,fxy=-3,fyy=-6y.x=0,y=0时,fxx=0,fxy=-3,fyy=0,(-3)^2-0>0,所以(0,0)不是极值点;x...