\[{F_{4}}\left(\alpha,\beta;\gamma,\alpha+\beta-\gamma+1;x(1-y),y(1-x)\right)={{%
}_{2}F_{1}}\left({\alpha,\beta\atop\gamma};x\right){{}_{2}F_{1}}\left({\alpha,%
\beta\atop\alpha+\beta-\gamma+1};y\right).\]