Work done by gas in adiabatic process

In an adiabatic process PV^γ=constant

Now I thought the work done by an ideal gas in an adiabatic process was given by the equation here: http://hyperphysics.phy-astr.gsu.edu...rmo/adiab.html

But while doing a GRE question the answer was given as P_fV_f - P_iV_i / 1 - γ

Is this correct (it must be!)? And if so why haven't I seen this equation anywhere before?