6x^2+4kx+8=0
两边同除以6,得x^2+(2/3)kx+(4/3)=0
用(a+b)^2=a^2+2ab+b^2,得x^2+2*x*(k/3)+(k/3)^2=0
所以(k/3)^2=4/3
得3k^2=36
k^2=12
k=根号12=2根号3