# Properties of Logarithms

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Properties of Logarithms
During this lesson, you will: Expand the logarithm of a product, quotient, or power Simplify (condense) a sum or difference of logarithms Mrs. McConaughy Honors Algebra 2

Part 1: Expanding Logarithms
Mrs. McConaughy Honors Algebra 2

The Product Rule PROPERTY: The Product Rule (Property)
Let M, N, and b be any positive numbers, such that b ≠ 1. log b (M ∙ N ) = log b M+ log b N The logarithm of a product is the sum of the logarithms. Connection: When we multiply exponents with a common base, we add the exponents. Mrs. McConaughy Honors Algebra 2

Example Expanding a Logarithmic Expression Using Product Rule
log (4x) = log 4 + log x is The logarithm of a product The sum of the logarithms. Use the product rule to expand: log4 ( 7 • 9) = _______________ log ( 10x) = ________________ = ________________ log4 ( 7) + log 4(9) log ( 10) + log (x) 1 + log (x) Mrs. McConaughy Honors Algebra 2

Property: The Quotient Rule (Property)
Let M, N, and b be any positive numbers, such that b ≠ 1. log b (M / N ) = log b M - log b N The logarithm of a quotient is the difference of the logarithms. Connection: When we divide exponents with a common base, we subtract the exponents. Mrs. McConaughy Honors Algebra 2

Example Expanding a Logarithmic Expression Using Quotient Rule
log (x/2) = log x - log 2 is The logarithm of a quotient The difference of the logarithms. Use the quotient rule to expand: log7 ( 14 /x) = ______________ log ( 100/x) = ______________ = ______________ log7 ( 14) - log 7(x) log ( 100) - log (x) 2 - log (x) Mrs. McConaughy Honors Algebra 2

PROPERTY: The Power Rule (Property)
Let M, N, and b be any positive numbers, such that b ≠ 1. log b Mx = x log b M When we use the power rule to “pull the exponent to the front,” we say we are _________ the logarithmic expression. expanding Mrs. McConaughy Honors Algebra 2

Example Expanding a Logarithmic Expression Using Power Rule
Use the power rule to expand: log5 74= _______________ log √x = ________________ = ________________ 4log5 7 log x 1/2 1/2 log x Mrs. McConaughy Honors Algebra 2

Summary: Properties for Expanding Logarithmic Expressions
Properties of Logarithms Let M, N, and b be any positive numbers, such that b ≠ 1. Product Rule: Quotient Rule: Power Rule: log b (M ∙ N ) = log b M+ log b N log b (M / N ) = log b M - log b N log b Mx = x log b M NOTE: In all cases, M > 0 and N >0. Mrs. McConaughy Honors Algebra 2

Check Point: Expanding Logarithmic Expressions
Use logarithmic properties to expand each expression: logb x2√y b. log6 3√x 36y4 log b x2 + logb y1/2 log 6 x1/3 - log636y4 2log b x + ½ logb y log 6 x1/3 - (log636 + log6y4) 1/3log 6 x - log log6y 2 Mrs. McConaughy Honors Algebra 2

Check Point: Expanding Logs
NOTE: You are expanding, not condensing (simplifying) these logs. Check Point: Expanding Logs Expand: log 2 3xy2 log 8 26(xy)2 = log log 2 x + 2log 2 y = log log 8 x2 + log 8 y2 = 6log log 8 x + 2log 8 y Mrs. McConaughy Honors Algebra 2

Part 2: Condensing (Simplifying) Logarithms
Mrs. McConaughy Honors Algebra 2

Part 2: Condensing (Simplifying) Logarithms
To condense a logarithm, we write the sum or difference of two or more logarithms as single expression. NOTE: You will be using properties of logarithms to do so. Mrs. McConaughy Honors Algebra 2

Properties for Condensing Logarithmic Expressions (Working Backwards)
Properties of Logarithms Let M, N, and b be any positive numbers, such that b ≠ 1. Product Rule: Quotient Rule: Power Rule: log b M+ log b N = log b (M ∙ N) log b M - log b N = log b (M /N) x log b M = log b Mx Mrs. McConaughy Honors Algebra 2

Example Condensing Logarithmic Expressions
Write as a single logarithm: log4 2 + log 4 32 = = log (4x - 3) – log x = log 4 64 3 log (4x – 3) x Mrs. McConaughy Honors Algebra 2

NOTE: Coefficients of logarithms must be 1 before you condense them using the product and quotient rules. Write as a single logarithm: ½ log x + 4 log (x-1) 3 log (x + 7) – log x c. 2 log x + log (x + 1) = log x ½ + log (x-1)4 = log √x (x-1)4 = log (x + 7)3 – log x = log (x + 7)3 x = log x2 + log (x + 1) = log x2 (x + 1) Mrs. McConaughy Honors Algebra 2

Check Point: Simplifying (Condensing) Logarithms
log log 3 4 = b. 3 log 2 x + log 2 y = c. 3log 2 + log 4 – log 16 = log 3 (20/4) = log 3 5 log 2 x 3y log 23 + log 4 – log 16 = log 32/16 =log 2 Mrs. McConaughy Honors Algebra 2

Example 1 Identifying the Properties of Logarithms
Sometimes, it is necessary to use more than one property of logs when you expand/condense an expression. Example 1 Identifying the Properties of Logarithms State the property or properties used to rewrite each expression: Quotient Rule (Property) Property:____________________________ log log 2 4 = log 2 8/4 = log 2 2 = 1 log b x3 y = log b x3 + log b 7 = 3log b x + log b 7 log log 5 6 = log 512 Product Rule/Power Rule Product Rule (Property) Mrs. McConaughy Honors Algebra 2

Example Demonstrating Properties of Logs
Use log 10 2 ≈ and log 10 3 ≈ to approximate the following: a. log 10 2/3 b. log c. log 10 9 log10 2 – log10 3 0.031 – 0.477 0.031 – 0.477 – 0.466 Mrs. McConaughy Honors Algebra 2

Homework Assignment: Properties of Logs Mrs. McConaughy
Honors Algebra 2

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