http://www.cbe21.com/subject/maths/html/040401/2002_09/20020911_1793.html
http://www.cbe21.com/subject/maths/html/040401/2002_09/20020911_1793.html上面那位给的是比较全了,最简单的是构造一个函数,通过判别式得证.