log8 3=a ==> 8^a=3
log3 5=b ==> 3^b=5
8^(ab)=5
2^(3ab)=5
∴log2 5=3ab
2^(3ab +1)=10
所以log2 10=3ab +1
∵lg 5= log10 5=(log2 5)/(log2 10)=3ab/(3ab+1)