@Carl Love Thank you for the clarification of expressions and procedures. 

Your code seems too full of programming skills to me. I believe it will help when I get used to this language

@tomleslie yes, you are right, there is no x_4.

I compare your code with mine and find out it's not 'f', but 'f(x)'. I have to complain that the error info created by Maple make no sense at all.

I also find that an initial point where x_2 not equal zero must be given, then maple can calculate the right answer. But since this answer is so wrong that I don't understand why maple doesn't check the answer by itself(I tried Mathmatica and it doesn't need the initial point)?

@tomleslie Thank you very very much. You help me a lot. Sorry, I don't have any reputation now. I will vote for you when I have.