6^a = 8
两边取以6为底的对数:
a = log(6)8 = log(6)2^3 = 3log(6)2 = 3log(6)(6/3)
= 3[log(6)6 - log(6)3]
= 3[1 - log(6)3)
log(6)3 = 1 -a/3