lim((x,y)→(0,0)) (xy)^2/(x^2+y^2)换元，x=ρcosθ，y=ρsinθ=lim(ρ→0) (ρ^2sinθcosθ)^2/ρ^2=lim ρ^2*(sinθcosθ)^2因为，ρ^2→0，(sinθcosθ)^2有界因此=0有不懂欢迎追问
