x1 = [-b+√(b^2-4ac)]/2a
x2 = [-b-√(b^2-4ac)]/2a
则x1+x2=-b/a；x1*x2=c/a （韦达定理）
x = {-(-4m)(+-)√[(-4m)^2 - 4*4*(m+2)]}/(2*4)
= [4m(+-)√(16m^2 -16m -32)]/8
= m(+-)√(m^2 -1m -2)
又x1^2+x2^2 = x1^2+x2^2 (+2x1x2 -2x1x2)
=(x1+x2)^2 -2x1x2 = (-b/a)^2 -2(c/a) = (4m/4)^2 -2[(m-2)/4)
=m^2 -(1/2)m +1
=m^2 -2(1/4)m +(1/4)^2 -1/16 +1
=(m-1/4)^2 +15/16
最小值就是（m-1/4）^2 =0的时候（没有更小的了……）
m=1/4
x1x2都比1/2大,可知x1+x2大于1
x1+x2=-b/a=m>1