# Basic Rules for Logarithms

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.

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