(x^2+y^2)dx+2xydy=0怎么求通解!

(x^2+y^2)dx+2xydy=0怎么求通解!
(x^2+y^2)dx+2xydy=0怎么求隐式通解!能不能通俗点!谢谢!我看过答案是
（x^2+y^2)dx+2xydy=0
x^2dx＋(y^2dx+2xydy)＝0
d(x^3/3)＋d(xy^2)＝0
d(x^3/3+xy^2)＝0
x^3/3+xy^2＝C
可是这两步
x^2dx＋(y^2dx+2xydy)＝0
d(x^3/3)＋d(xy^2)＝0
是怎么变的!