log4(15)+log5(4)=log4(3)+log4(5)+log5(4)≥log4(3)+2√［log4(5)*log5(4)］=log4(3)+2,log2(3)+log3(8)+log4(3)=3/2log2(3)+3log3(2)≥2√［3/2log2(3)*3log3(20)]=3√2，∴原式≥3√2+2>6。
