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Answer:
log₅(21) = log₅(3) +log₅(7)
Explanation:
The relevant relation is ...
log(ab) = log(a) +log(b)
where the logs have any consistent base.
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Here, we have ,,,
ab = 21 = 3·7.
Then ...
log(21) = log(3) +log(7)
When the logs are to base 5, that becomes ...
log₅(21) = log₅(3) +log₅(7)