设A=P(x,y,z)i+Q(x,y,z)j+R(x,y,z)k, r=xi+yj+zk. 求证dA=(grad P·dr)i+(grad Q·dr)j+(grad R·dr)k.
设A=P(x,y,z)i+Q(x,y,z)j+R(x,y,z)k,
r=xi+yj+zk.
求证dA=(grad P·dr)i+(grad Q·dr)j+(grad R·dr)k.
其中dPi=grad P·dr.
同理dQ=gradQ·dr,dR=gradR·dr.
所以有
dA=(grad P·dr)i+(grad Q·dr)j+(grad R·dr)k.