Holt McDougal Algebra Properties of Logarithms 4-4 Properties of Logarithms Holt Algebra 2 Warm Up Warm Up Lesson Presentation Lesson Presentation Lesson Quiz Lesson Quiz Holt McDougal Algebra 2

4-4 Properties of Logarithms Use properties to simplify logarithmic expressions. Translate between logarithms in any base. Objectives

Holt McDougal Algebra Properties of Logarithms Because logarithms are exponents, you can derive the properties of logarithms from the properties of exponents

Holt McDougal Algebra Properties of Logarithms Remember that to multiply powers with the same base, you add exponents.

Holt McDougal Algebra Properties of Logarithms Express log log 6 9 as a single logarithm. Simplify. Example 1: Adding Logarithms 2 To add the logarithms, multiply the numbers. log log 6 9 log 6 (4  9) log 6 36 Simplify. Think: 6 ? = 36.

Holt McDougal Algebra Properties of Logarithms Express as a single logarithm. Simplify, if possible. 6 To add the logarithms, multiply the numbers. log log 5 25 log 5 (625 25) log 5 15,625 Simplify. Think: 5 ? = Check It Out! Example 1a

Holt McDougal Algebra Properties of Logarithms Express as a single logarithm. Simplify, if possible. –1 To add the logarithms, multiply the numbers. Simplify. Check It Out! Example 1b Think: ? = log (27 ) log 3 log 27 + log

Holt McDougal Algebra Properties of Logarithms Remember that to divide powers with the same base, you subtract exponents Because logarithms are exponents, subtracting logarithms with the same base is the same as finding the logarithms of the quotient with that base.

Holt McDougal Algebra Properties of Logarithms The property above can also be used in reverse. Just as a 5 b 3 cannot be simplified, logarithms must have the same base to be simplified. Caution

Holt McDougal Algebra Properties of Logarithms Express log – log 5 4 as a single logarithm. Simplify, if possible. Example 2: Subtracting Logarithms To subtract the logarithms, divide the numbers. log – log 5 4 log 5 (100 ÷ 4) 2 log 5 25 Simplify. Think: 5 ? = 25.

Holt McDougal Algebra Properties of Logarithms Express log 7 49 – log 7 7 as a single logarithm. Simplify, if possible. To subtract the logarithms, divide the numbers log 7 49 – log 7 7 log 7 (49 ÷ 7) 1 log 7 7 Simplify. Think: 7 ? = 7. Check It Out! Example 2

Holt McDougal Algebra Properties of Logarithms Because you can multiply logarithms, you can also take powers of logarithms.

Holt McDougal Algebra Properties of Logarithms Express as a product. Simplify, if possible. Example 3: Simplifying Logarithms with Exponents A. log B. log log (5) = 30 20log ( ) = Because 8 = 4, log 8 4 = Because 2 5 = 32, log 2 32 = 5.

Holt McDougal Algebra Properties of Logarithms Express as a product. Simplify, if possibly. a. log10 4 b. log log10 4(1) = 4 2log (2) = 4 Because 5 2 = 25, log 5 25 = 2. Because 10 1 = 10, log 10 = 1. Check It Out! Example 3

Holt McDougal Algebra Properties of Logarithms Express as a product. Simplify, if possibly. c. log 2 ( ) 5 5(–1) = –5 5log 2 ( ) Check It Out! Example 3 Because 2 –1 =, log 2 = –

Holt McDougal Algebra Properties of Logarithms Exponential and logarithmic operations undo each other since they are inverse operations.

Holt McDougal Algebra Properties of Logarithms Example 4: Recognizing Inverses Simplify each expression. b. log 3 81 c. 5 log 5 10 a. log log log 3 3  3  3  3 log log

Holt McDougal Algebra Properties of Logarithms b. Simplify 2 log 2 (8x) a. Simplify log x8x Check It Out! Example 4 log log 2 (8x)

Holt McDougal Algebra Properties of Logarithms Most calculators calculate logarithms only in base 10 or base e (see Lesson 7-6). You can change a logarithm in one base to a logarithm in another base with the following formula.

Holt McDougal Algebra Properties of Logarithms Example 5: Changing the Base of a Logarithm Evaluate log Method 1 Change to base 10 log 32 8 = log8 log ≈ ≈ 0.6 Use a calculator. Divide.

Holt McDougal Algebra Properties of Logarithms Evaluate log Method 1 Change to base 10. log 9 27 = log27 log ≈ ≈ 1.5 Use a calculator. Divide. Check It Out! Example 5a

Holt McDougal Algebra Properties of Logarithms Evaluate log Method 1 Change to base 10. Log 8 16 = log16 log ≈ ≈ 1.3 Use a calculator. Divide. Check It Out! Example 5b

Holt McDougal Algebra Properties of Logarithms Lesson Quiz: Part I Express each as a single logarithm. 1. log log 6 24 log = 3 2. log – log 3 4 Simplify. 3. log ,000 log 3 27 = 3 30, log 4 4 x –1 x – 1 6. log log

Holt McDougal Algebra Properties of Logarithms Lesson Quiz: Part II Use a calculator to find each logarithm to the nearest thousandth. 7. log How many times as much energy is released by a magnitude-8.5 earthquake as a magntitude- 6.5 earthquake? – log