∫(0,1/2)∫(0,1)4xydydx =4∫(0,1/2)1/2xy^2|(0,1)dx =2∫(0,1/2)x(1^2-0^2)dx =2∫(0,1/2)xdx =2×1/2x^2|(0,1/2) =(1/2)^2-0^2 =1/4
