x1/x2 + x2/x1
=(x1^2 + x2^2)/(x1x2)
=[(x1 + x2)^2 - 2x1x2]/(x1x2)x1-x2呢。x1-x2有两个值，一正一负 (x1 - x2)^2 = (x1 + x2)^2 - 4x1x2然后开根号