Help with this example problem

[aruna1]

Homework Statement

hello
this is a example problem in book "Probability, Random Variables and Stochastic Processes 4th ed - Papoulis" pg 183

i am wondering how particular value came.i havemarked with red circle it in attached pdf file.
please take a look
thanks

Homework Equations

The Attempt at a Solution

 

[HallsofIvy]

They are calculating the area of the shaded region. The area of region S, is $A= \int_S\int dA$ which, in Cartesian coordinates, is $\int_S\int dxdy$. Of course dxdy= 1 dxdy.

 

[aruna1]

They are calculating the area of the shaded region. The area of region S, is $A= \int_S\int dA$ which, in Cartesian coordinates, is $\int_S\int dxdy$. Of course dxdy= 1 dxdy.

well shouldn't it be as following pdf?
x=z-y
dx=dz