(x1+x2)^2-4x1x2=(x1-x2)^2
x1-x2 =√[(x1+x2)^2-4x1*x2]
(√x1-√x2)^2=x1+x2-2√(x1*x2)
√x1-√x2=√[x1+x2-2√(x1*x2)]