x+y=1
A(1,0) B(0,1)
∫[D]∫e^(x+y)dxdy=4∫[0,1]e^xdx∫[0,1-x]e^ydy
=4∫[0,1]e^x[e^(1-x)-1]dx
=4∫[0,1](e-e^x)dx
=4*(ex-e^x)|[0,1]
=4