log2（log8^m)=log8(log2^m),
log2（1/3log2^m)=log8(log2^m)=1/3log2(log2^m)
1/3log2^m=(log2^m)^1/3
[1/27(log2^m)^2-1][log2^m]=0
(log2^m)^2=27或0