依据求根公式得：x=[-b±√b^2-4ac]/2a
∴|x1-x2|=[√b^2-4ac]/|a|
x1^3+x2^3
=(x1+x2)(x1^2+x2^2-x1x2)
=-(b/a)[(x1+x2)^2-3x1x2]
=-(b/a)[(b/a)^2-3c/a]
=-(b/a)^3+3bc/a^2