X 和Y 相互独立-->f(x,y)=f(x)*f(y)=(1/2)e^(-x/2)*(1/4)e^(-y/4)
p(X>Y)=∫∫f(x,y)dxdy(积分区域为y=0,y=x所围面积)
=∫(0-->∞)(1/4)e^(-y/4)dy∫(y-->∞)(1/2)e^(-x/2)dx
=1/3