Changing base of Log to 10

[jonboy]

When are you allowed to change the base of the log to 10? I looked on SOS Math and I can't find it. Can someone please tell me or show me the rule? Thanks.

 

[morson]

I am not sure what you mean. Are you referring to the rule:

loga(b) = logc(b)/logc(a) ?

Or, in the case when c = b:

loga(b) = 1/logb(a) ?

Only works for positive a, b and c, by the way.