Let x = a+b y = a-b.
x+Y = 2a & gt； =0
x-Y = 2b & gt； =0
0 = & ltx = a+b & lt； =2
-2 = & lt； y = a-b & lt； =2
The shadow area drawn by the above four inequalities is a triangle:
s= 1/2X4X2=4