@Kitonum Thank you so much, It is so helpful! I got it

@Carl Love Thank you very much, Just one more question: why some arrows is so weird?How can we analyze it to know threre is a closed orbit? 

@Carl Love Thank you very much. I got it

@Preben Alsholm Thank you every much, I got it but I don't know how to deal with the rootof(the fourth solution), it asks me to get some condition to let the solution stable or nonstable

Thank you very much, It is heipful