In this paper, we prove the following reversed Hardy-Littlewood-Sobolev inequality with extended kernel begin{equation*} int_{mathbb{R}_+^n}int_{partialmathbb{R}^n_+} frac{x_n^beta}{|x-y|^{n-alpha}}f(y)g(x) dydxgeq C_{n,alpha,beta,p}|f|_{L^{p}(partialmathbb{R}_+^n)} |g|_{L^{q}(mathbb{R}_+^n)} end{equation*} for any nonnegative functions $fin L^{p}(partialmathbb{R}_+^n)$ and $gin L^{q}(mathbb{R}_+^n)$, where $ngeq2$, $p, qin (0,1)$, $alpha>n$, $0leqbeta<frac{alpha-n}{n-1}$, $p>frac{n-1}{alpha-1-(n-1)beta}$ such that $frac{n-1}{n}frac{1}{p}+frac{1}{q}-frac{alpha+beta-1}{n}=1$. We prove the existence of extremal functions for the above inequality. Moreover, in the conformal invariant case, we classify all the extremal functions and hence derive the best constant via a variant method of moving spheres, which can be carried out emph{without lifting the regularity of Lebesgue measurable solutions}. Finally, we derive the sufficient and necessary conditions for existence of positive solutions to the Euler-Lagrange equations by using Pohozaev identities. Our results are inspired by Hang, Wang and Yan cite{HWY}, Dou, Guo and Zhu cite{DGZ} for $alpha<n$ and $beta=1$, and Gluck cite{Gl} for $alpha<n$ and $betageq0$.
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