In this paper, we will solve the following nonlinear elliptic equation with weight. First we are concerned with properties of positive solutions of the following fractional elliptic systemwhere N≥3,0≤β<α<N, p, q>1and p+q<N+α-β/N-α+β. We show that positive solutions of (1.1) are radially symmetric and belong to L∞(Rn). Moreover, if α=2,β=0,p≤q, we show that positive solution pair (u,v) of (1.1) is unique and u=v=U, where U is the unique positive solution of the problem-Δu+u=up+q in RN. Second, we are concerned with the existence of positive solution of the following nonlinear elliptic involving critical Hardy-Sobolev exponentwhere RN=Rk×RN-k, with2≤k<N,λ>0and x=(y, z)∈Rk x RN-k. For a given real numbers s, such that Suppose that Ω is a C1bounded domain in RN with0∈(?)Ω,(?)Ω is C2at0, and the principal curvature of (?)Ω at0is nonpositive,but do not all vanish. We may also suppose RN-k∩Ω=φ,RN-κ∩(?)Ω≠φ.We can also prove that the following equation has a positive solution if λ>0,1≤p<N/N-2, and the equation has a positive solution if λ>0,1<p<N+2/N-2. |