Hokay, just clearing this up a bit, and adding a few more rules.
Basics first:
log<sub>b</sub>MN = log<sub>M</sub> + log<sub>b</sub>N
log<sub>b</sub>M/N = log<sub>M</sub> - log<sub>b</sub>N
log<sub>b</sub>b = 1
log<sub>b</sub>1 = 0
log<sub>b</sub>b<sup>k</sup> = k
What Keypad posted, but tidied up a bit (easier to read):
Log<sub>a</sub>b<sup>c</sup> = c * Log<sub>a</sub>b
Log<sub>a</sub>b = Log<sub>c</sub>b / Log<sub>c</sub>a
Log<sub>a</sub>b * Log<sub>b</sub>a = 1
a<sup>1/Log_b[a]</sup> = b
a = b<sup>Log_b[a]</sup>
YAY to the QUICKEDIT feature.
and if i made a mistake, please let me know.
Learn all those by heart, you'll have a lot less difficulty. And no, learn off by heart doesn't mean rote learn all of them. Try to understand em. As in think about it for a bit, 'understand' it, and you won't forget it. Logs laws aren't all that hard to learn. Unlike trig rules. :-\