∂u/∂x=2x ∂u/∂y=2y ∂u/∂z=2zgradu(0,1,2)=(0,2,4)а方向的单位向量=(-1/√2,-1/√2,0)故 ∂u/∂l=(0,2,4)(-1/√2,-1/√2,0)=-√2