顶点(0,0),(2,4)的连线方程：x=y/2
顶点(6,0),(2,4)的连线方程：x=6-y
D∫∫ye^xdxdy
=[0,4]∫ydy [y/2,6-y]∫e^xdx
=[0,4]∫y[e^(6-y)-e^(y/2)]dy
=[0,4]∫{e^6*ye^(-y)-ye^(y/2)}dy
=e^6 -9e^2 -4