设f(x,y)具一阶连续偏导数,且满足x•(df/dx)+y•(df/dy)=0.证明f((x,y)在极坐标下与向量r无关 参考知识1 做变化x=rcost ,y=rsint df/dx=(1/cost)df/dr-[1/(rsint)]df/dt df/dy=(1/sint)df/dr-[1/(rcost)]df/dt x(df/dx)+y(df/dy)=2rdf/dr=0 df/dr=0 f和r无关