log7(9)=log(9)/log(7)=(log(6)*log(9))/(log(6)*log(7))
log6(8)=log(8)/log(6)=(log(7)*log(8))/(log(6)*log(7))
因为：log(6)*log(9)=0.742545
log(7)*log(8)=0.7631996
所以：log6(8)>log7(9)