m=2时
-8x-2=0
x=-1/4,为负根
m≠2时
△=16m^2-4(m-2)(2m-6)=8m^2+40m-48=8(m+6)(m-1) ≥0
m≥1,或,m≤-6
x1+x2=4m/(m-2)