(2)a2-2xyy2yx-(x+y3d=0; (3)kax+(re-2y)=0; (4)(rcos y+cos x)y'-ysinx+siny=0 (5)(x2-y)dx-xdy=0; (o(x-2y)dx-x dy=0 (7)(1+edo+22oda0 8(r +y )dx+ydy=0 2.利用观察法求出下列方程的积分因子,并求其通解: (1)(x+y)(dx-dy)=dx+dy;