fx(0,0)=lim(x->0)[f(x,0)-f(0,0)]/x
=lim(x->0)[√x² - 0]/x
=lim(x->0)|x|/x,不存在
fy(0,0)=lim(y->0)[f(0,y)-f(0,0)]/y
=lim(y->0)[√y^4] / y
=0 ,存在
fx(0,0)=lim(x->0)[f(x,0)-f(0,0)]/x
=lim(x->0)[√x² - 0]/x
=lim(x->0)|x|/x,不存在
fy(0,0)=lim(y->0)[f(0,y)-f(0,0)]/y
=lim(y->0)[√y^4] / y
=0 ,存在