一元2次方程两实数根的关系：ax2+bx+c=0 (a≠0),x1+x2=-b/a,x1*x2=c/a
x1+x2=-2(k+1)/k,x1*x2=-(3k-1)/k
-2(k+1)/k=-(3k-1)/k
解得k=3