Exponential and Logarithmic Functions Copyright Cengage Learning All

Exponential and Logarithmic Functions Copyright © Cengage Learning. All rights reserved.

4. 4 Laws Of Logarithms Copyright © Cengage Learning. All rights reserved.

Objectives ► Laws of Logarithms ► Expanding and Combining Logarithmic Expressions ► Change of Base Formula 3

Laws of Logarithms 4

Laws of Logarithms Since logarithms are exponents, the Laws of Exponents give rise to the Laws of Logarithms. 5

Example 1 – Using the Laws of Logarithms to Evaluate Expressions Evaluate each expression. (a) log 4 2 + log 4 32 (b) log 2 80 – log 2 5 (c) log 8 Solution: (a) log 4 2 + log 4 32 = log 4(2 32) = log 4 64 = 3 Law 1 Because 64 = 43 6

Example 1 – Solution (b) log 2 80 – log 2 5 = log 2 (b) (c) cont’d Law 2 16 = 24 = log. Because 16 = 4 2 log 8 = log 8– 1/3 = log – 0. 301 Law 3 Property of negative exponents Calculator 7

Expanding and Combining Logarithmic Expressions 8

Expanding and Combining Logarithmic Expressions The Laws of Logarithms allow us to write the logarithm of a product or a quotient as the sum or difference of logarithms. This process, called expanding a logarithmic expression, is illustrated in the next example. 9

Example 2 – Expanding Logarithmic Expressions Use the Laws of Logarithms to expand each expression. (a) log 2(6 x) (b) log 5(x 3 y 6) (c) ln Solution: (a) log 2(6 x) = log 2 6 + log 2 x Law 1 (b) log 5(x 3 y 6) = log 5 x 3 + log 5 y 6 Law 1 = 3 log 5 x + 6 log 5 y Law 3 10

Example 2 – Solution (c) = ln(ab) – cont’d Law 2 = ln a + ln b – ln c 1/3 Law 1 = ln a + ln b – ln c Law 3 11

Expanding and Combining Logarithmic Expressions The Laws of Logarithms also allow us to reverse the process of expanding that was done in Example 2. That is, we can write sums and differences of logarithms as a single logarithm. This process, called combining logarithmic expressions, is illustrated in the next example. 12

Example 3 – Combining Logarithmic Expressions Combine 3 log x + log (x + 1) into a single logarithm. Solution: 3 log x + log(x + 1) = log x 3 + log(x + 1)1/2 = log(x 3(x + 1)1/2) Law 3 Law 1 13

Expanding and Combining Logarithmic Expressions Logarithmic functions are used to model a variety of situations involving human behavior. One such behavior is how quickly we forget things we have learned. For example, if you learn algebra at a certain performance level (say, 90% on a test) and then don’t use algebra for a while, how much will you retain after a week, a month, or a year? Hermann Ebbinghaus (1850– 1909) studied this phenomenon and formulated the law described in the next example. 14

Example 5 – The Law of Forgetting If a task is learned at a performance level P 0, then after a time interval t the performance level P satisfies log P = log P 0 – c log(t + 1) where c is a constant that depends on the type of task and t is measured in months. (a) Solve for P. (b) If your score on a history test is 90, what score would you expect to get on a similar test after two months? After a year? (Assume that c = 0. 2. ) 15

Example 5 – Solution (a) We first combine the right-hand side. log P = log P 0 – c log(t + 1) Given equation log P = log P 0 – log(t + 1)c Law 3 log P = Law 2 P= Because log is one-to-one 16

Example 5 – Solution cont’d (b) Here P 0 = 90, c = 0. 2, and t is measured in months. In two months: t=2 and In one year: t = 12 and Your expected scores after two months and one year are 72 and 54, respectively. 17

Change of Base Formula 18

Change of Base Formula For some purposes we find it useful to change from logarithms in one base to logarithms in another base. Suppose we are given logax and want to find logbx. Let y = logb x 19

Change of Base Formula We write this in exponential form and take the logarithm, with base a, of each side. by = x loga (by) = loga x yloga b = loga x Exponential form Take loga of each side Law 3 Divide by logab 20

Change of Base Formula This proves the following formula. In particular, if we put x = a, then loga a = 1, and this formula becomes 21

Example 6 – Evaluating Logarithms with the Change of Base Formula Use the Change of Base Formula and common or natural logarithms to evaluate each logarithm, correct to five decimal places. (a) log 85 (b) log 920 Solution: (a) We use the Change of Base Formula with b = 8 and a = 10: log 8 5 = 0. 77398 22

Example 6 – Solution cont’d (b) We use the Change of Base Formula with b = 9 and a = e: log 9 20 = 1. 36342 23