x1=(-b+√Δ)/2a
x2= (-b-√Δ)/2a
x1-x2绝对值=(-b+√Δ+b+√Δ)/2a=2√Δ/2a,因为绝对值为非负数,所以a带绝对值符合
x1-x2=√(x1-x2)²
=√[(x1+x2)²-4x1·x2]
=√[(-b/a)²-(4c/a)]
=√[(b²-4ac)/(a²)]
=[√(b²-4ac)]/|a|
(x1-x2)^2=(x1+x2)^2-4x1x2=(-b/a)^2-4c/a=(b^2-4ac)/a^2
韦达定理,平方再开方
根与系数的关系~~