应该是L=OA+AH,直接计算：∫(x+y)dx-(x-y)dy=∫OA(x+y)dx-(x-y)dy+∫AH(x+y)dx-(x-y)dy=∫(0,1)(x)dx+∫(0,1)(-1+y)dy=1/2-1+1/2=0
