解答题 28.设Q(x，y)在平面xOy上具有一阶连续的偏导数，且∫_L2xydx+Q(x，y)dy与路径无关，且对任意的t有∫_(0,0)^(t,1)2xydx+Q(x，y)dy=∫_(0,0)^(1,t)2xydx+Q(x，y)dy，求Q(x，y)．