A thermodynamic system under goes cyclic process ABCDA as shown in figure. The work done by the system

WBCOB = - Area of triangle BCO = -P0V0/ 2=negative since cycle is anti clock wise WAODA = + Area of triangle AOD = + P0V0 / 2=positive since cycle is clock wise (here each triangle area is half of rectangle area) work done by the system = sum of the areas under the curve=-P0V02+P0V02=0