Question #21 on the official GRE Physics Prep book: For an adiabatic process involving an ideal gas having volume V and temperature T , which of the following is constant? ( γ = CP/CV ) The answer is T*V^γ-1.

According to this website: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/adiab.html, P*V is constant but not with T. So what is the derivation? Can I get the answer from PV=nRT?