why change in volume is area * displacement??

• Mechanical Work, i.e., Pressure-Volume Work

volume = area x length

or

change in volume =change in( area x length)................(1)

but because area of piston is constant during the experiment

thus eq. (1) becomes

change in volume =  area x (change in length)....(2)

as we know 

change in length = displacement

so eqn (2) becomes..

change in volume =  area x displacement