x1的绝对值=2  
当X1、X2是一元二次方程aX^2+bX+c=0的两根时
|X1|+|X2|
=√[(|X1|+|X2|)^2]
=√[(X1+X2)^2+2|X1*X2|-2X1*X2]
=√[(-b/a)^2+2|c/a|-2c/a]
=√[(b^2+|2ac|-2ac)]/|a|.