x1+x2=-b/a
x1x2=c/a
1、
︱x1-x2｜
=√（x1-x2)²
=√(x1+x2)²-4x1x2
=√b²/a²-4c/a
=√(b²-4ac/a²)
（x1+x2）/2
=-b/2a
2、
x1^3+x2^3
=(x1+x2)(x1²+-x1x2+x2²)
=(x1+x2)((x1+x2)²-3x1x2)
=（-b/a）(（-b/a)²-3c/a)
=(-b/a)(b²-3ac/a²)
=(3abc-b³)/a³=√(x1+x2)²-4x1x2 这一步-4x1x2在不在根号下=√（(x1+x2)²-4x1x2）在