For the reaction
$N _{2}( g )+ O _{2}( g ) \rightleftharpoons 2 NO ( g )$
the equilibrium constant is $K _{1}$. The equilibrium constant is $K _{2}$ for the reaction
$2 NO ( g )+ O _{2}( g ) \rightleftharpoons 2 NO _{2}( g ) .$
What is $K$ for the reaction
$NO _{2}( g ) \rightleftharpoons 1 / 2 N _{2}( g )+ O _{2}( g ) ?$