${\mathrm{NO}}_2$ is a mixed oxide is proved by its reaction with NaOH, and forms

$2{\mathrm{NO}}_2+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{HNO}}_2+{\mathrm{HNO}}_3$

When dissolved in water, gives a mixture of nitrous acid and nitric acid. 

${\mathrm{N}}_2{\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HNO}}_2+{\mathrm{HNO}}_3$

${\mathrm{HNO}}_3+{\mathrm{HNO}}_2+2\mathrm{NaOH}\to {\mathrm{NaNO}}_2+{\mathrm{NaNO}}_3+2{\mathrm{H}}_2\mathrm{O}$