DEFINITION OF NOTATION: logb x = y is equivalent
to by = x
In other words, log b x = y means the same thing as by =
x
NOTE: In the rules below it is assumed that b > 0, b ≠ 1,
M > 0, N > 0, and p is any real number.
I. log b1 = 0
II. log bb = 1
III. log bbx = x
IV.
where x > 0 .
V. log b(M ٠ N) = logbM + logbN
VI.
VII.
[NOTE: This rule is often used when solving exponential equations.]
VIII. If log b x = logb y, then x = y . This is true
because f (x) = logb x is a 1 - 1 function.
IX.
[NOTE: This is known as the Change of Base Formula.]
X. If log b x = y , then by = x . This comes from the
basic definition at the top of this page.
XI. If bx = by , then x = y . This is true because f (x)
= logb x is a 1 - 1 function.
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NOTE: Rules III and IV are true becausef (x) = logb x and
f (x) = bx are inverses of each other.
Therefore,(g ◦ f )(x) = x and ( f ◦ g)(x) = x .
NOTE: Rules VIII and X are frequently used when solving
log equations.
NOTE: Rule XI is sometimes used when solving exponental
equations. It is easy to show that Rule XI
is true, because Rule III is true.