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

Warm Up Simplify. 1. (26)(28) 214 2. (3–2)(35) 33 3. 38 4. 44 5. (73)5 715 Write in exponential form. 6. logx x = 1 7. 0 = logx1 x1 = x x0 = 1

Objectives Use properties to simplify logarithmic expressions. Translate between logarithms in any base.

Remember that to multiply powers with the same base, you add exponents.

Example 1: Adding Logarithms Express log64 + log69 as a single logarithm. Simplify. log64 + log69 To add the logarithms, multiply the numbers. log6 (4  9) log6 36 Simplify. 2 Think: 6? = 36.

Check It Out! Example 1a Express as a single logarithm. Simplify, if possible. log5625 + log525 log5 (625 • 25) To add the logarithms, multiply the numbers. log5 15,625 Simplify. 6 Think: 5? = 15625

Express as a single logarithm. Simplify, if possible. Check It Out! Example 1b Express as a single logarithm. Simplify, if possible. log 27 + log 1 3 9 1 3 log (27 • ) 9 To add the logarithms, multiply the numbers. 1 3 log 3 Simplify. –1 Think: ? = 3 1 3

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.

The property above can also be used in reverse. Just as a5b3 cannot be simplified, logarithms must have the same base to be simplified. Caution

Example 2: Subtracting Logarithms Express log5100 – log54 as a single logarithm. Simplify, if possible. log5100 – log54 To subtract the logarithms, divide the numbers. log5(100 ÷ 4) log525 Simplify. 2 Think: 5? = 25.

Check It Out! Example 2 Express log749 – log77 as a single logarithm. Simplify, if possible. log749 – log77 To subtract the logarithms, divide the numbers log7(49 ÷ 7) log77 Simplify. 1 Think: 7? = 7.

Because you can multiply logarithms, you can also take powers of logarithms.

Example 3: Simplifying Logarithms with Exponents Express as a product. Simplify, if possible. A. log2326 B. log86420 6log232 20log84 6(5) = 30

Check It Out! Example 3 Express as a product. Simplify, if possibly. a. log104 b. log5252 4log10 2log525 Because 101 = 10, log 10 = 1. Because 52 = 25, log525 = 2. 4(1) = 4 2(2) = 4

Express as a product. Simplify, if possibly. Check It Out! Example 3 Express as a product. Simplify, if possibly. c. log2 ( )5 1 2 5log2 ( ) 1 2 Because 2–1 = , log2 = –1. 1 2 5(–1) = –5

Exponential and logarithmic operations undo each other since they are inverse operations.

Example 4: Recognizing Inverses Simplify each expression. a. log3311 b. log381 c. 5log510 log3311 log33  3  3  3 5log510 11 log334 10 4

Check It Out! Example 4 a. Simplify log100.9 b. Simplify 2log2(8x) log 100.9 2log2(8x) 0.9 8x

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.

Example 5: Changing the Base of a Logarithm Evaluate log328. Method 1 Change to base 10 log328 = log8 log32 0.903 1.51 ≈ Use a calculator. Divide. ≈ 0.6

Method 2 Change to base 2, because both 32 and 8 are powers of 2. Example 5 Continued Evaluate log328. Method 2 Change to base 2, because both 32 and 8 are powers of 2. log328 = log28 log232 = 3 5 Use a calculator. = 0.6

Check It Out! Example 5a Evaluate log927. Method 1 Change to base 10. 1.431 0.954 ≈ Use a calculator. ≈ 1.5 Divide.

Check It Out! Example 5a Continued Evaluate log927. Method 2 Change to base 3, because both 27 and 9 are powers of 3. log927 = log327 log39 = 3 2 Use a calculator. = 1.5

Check It Out! Example 5b Evaluate log816. Method 1 Change to base 10. 1.204 0.903 ≈ Use a calculator. Divide. ≈ 1.3

Check It Out! Example 5b Continued Evaluate log816. Method 2 Change to base 4, because both 16 and 8 are powers of 2. log816 = log416 log48 = 2 1.5 Use a calculator. = 1.3

Express each as a single logarithm. 1. log69 + log624 log6216 = 3 Lesson Quiz: Part I Express each as a single logarithm. 1. log69 + log624 log6216 = 3 2. log3108 – log34 log327 = 3 Simplify. 3. log2810,000 30,000 4. log44x –1 x – 1 5. 10log125 125 6. log64128 7 6

Use a calculator to find each logarithm to the nearest thousandth. Lesson Quiz: Part II Use a calculator to find each logarithm to the nearest thousandth. 7. log320 2.727 8. log 10 1 2 –3.322 9. How many times as much energy is released by a magnitude-8.5 earthquake as a magntitude-6.5 earthquake? 1000