At certain temperature compound.AB₂ (g) dissociates according to the reaction $2{\mathrm{AB}}_2\left(\mathrm{g}\right)\rightleftharpoons 2\mathrm{AB}\left(\mathrm{g}\right)+{\mathrm{B}}_2\left(\mathrm{g}\right)$ With degree of dissociation $\alpha$, which is small compared with unity. The expression of K _p in terms of $\alpha$ and initial pressure P is :