Conceptual Explanations: Logarithms

Transcription

1 Conceptual Eplanations: Logarithms Suppose you are a iologist investigating a population that doules every year. So if you start with 1 specimen, the population can e epressed as an eponential function: p(t)=2 t where t is the numer of years you have een watching, and p is the population. Question: How long will it take for the population to eceed 1,000 specimens? We can rephrase this question as: 2 to what power is 1,000? This kind of question, where you know the ase and are looking for the eponent, is called a logarithm. log (read, the logarithm, ase two, of a thousand ) means 2, raised to what power, is 1000? In other words, the logarithm always asks What eponent should we use? This unit will e an eploration of logarithms. Important note. The suscript 2 in log 2 doesn t multiply anything; it just says what numer is eing raised to a power to get Don t try to multiply it, divide it, or cancel it! 0BA few quick eamples to start things off UProlemU UMeansU UThe answer isu Uecause log to what power is 8? is 8 log to what power is 16? is 16 log to what power is 10? somewhere etween 3 and =8 and 2 4 =16 log to what power is 2? ⅓ 8 ⅓ = 3 8=2 log 10 10, to what power is 10,000? =10, log to what power is? = = log to what power is 0? There is no answer 5 something will never e 0 As you can see, one of the most important parts of finding logarithms is eing very familiar with how eponents work! 1BThe logarithm eplained y analogy to roots The logarithm may e the first really new concept you ve encountered in Algera II. So one of the easiest ways to understand it is y comparison with a familiar concept: roots. Suppose someone asked you: Eactly what does root mean? You do understand roots, ut they are difficult to define. After a few moments, you might come up with a definition very similar to the question definition of logarithms given aove. 3 8 means what numer cued is 8? Chapter 6: Logarithms Conceptual Eplanations 1/8

2 Now the person asks: How do you find roots? Well...you just play around with numers until you find one that works. If someone asks for 25, you just have to know that 5 2 =25. If someone asks for 30, you know that has to e igger than 5 and smaller than 6; if you need more accuracy, it s time for a calculator. All that information aout roots applies in a very analogous way to logarithms. URoots ULogs UThe question a means what numer, raised to the a power, is? As an equation,? a = UEample that comes out even 3 8 =2 log 2 8=3 log a means a, raised to what power, is? As an equation, a? = UEample that doesn t 3 10 is a it more than 2 log 2 10 is a it more than 3 UOut of domain eample 4 does not eist ( 2 will never give 4) log 2 (0) and log 2 ( 1) do not eist (2 will never give 0 or a negative answer) 2BRewriting logarithm equations as eponent equations Both root equations and logarithm equations can e rewritten as eponent equations. 9 3 can e rewritten as U3 2 =9U. These two equations are the same statement aout numers, written in two different ways. 9 asks the question What numer squared is 9? So the equation 9 =3 asks this question, and then answers it: 3 squared is 9. We can rewrite logarithm equations in a similar way. Consider this equation: log 3 (⅓)= 1 If you are asked to rewrite that logarithm equation as an eponent equation, think aout it this way. The left side asks: 3 to what power is ⅓? And the right side answers: 3 to the 1 power is ⅓. 3 1 =⅓. These two equations, Ulog 3 (⅓)= 1U and U3 1 =⅓U, are two different ways of epressing the same numerical relationship. Chapter 6: Logarithms Conceptual Eplanations 2/8

3 3BThe logarithm defined as an inverse function can e defined as the inverse function of 2. Recall the definition of an inverse function f 1 () is defined as the inverse of f() if it reverses the inputs and outputs. So we can demonstrate this inverse relationship as follows: is the inverse function of Similarly, log 2 is the inverse function of the eponential function 2. log 2 is the inverse function of log 2 3 (You may recall that during the discussion of inverse functions, 2 was the only function you were given that you could not find the inverse of. Now you know!) In fact, as we noted in the first chapter, is not a perfect inverse of 2, since it does not work for negative numers. ( 3) 2 =9, ut 9 is not 3. Logarithms have no such limitation: log 2 is a perfect inverse for 2. The inverse of addition is sutraction. The inverse of multiplication is division. Why do eponents have two completely different kinds of inverses, roots and logarithms? Because eponents do not commute. 3 2 and 2 3 are not the same numer. So the question what numer squared equals 10? and the question 2 to what power equals 10? are different questions, which we epress as 10 and log 2 10, respectively, and they have different answers. 2 and 2 are not the same function, and they therefore have different inverse functions and log 2. 4BFormal Definition of the Logarithm The definition log is the inverse of can e written in formal mathematical notation as log or log ( ) (the triple equals sign means is defined as ). Convince yourself that these formulas really are saying log is the inverse of other properties of logarithms can e derived from them.. All the 5BProperties of Logarithms Just as there are three fundamental laws of eponents, there are three fundamental laws of logarithms. Chapter 6: Logarithms Conceptual Eplanations 3/8

4 log (a) = log a + log log a = log a log log (a ) = log a As always, these algeraic generalizations hold for any a,, and. Eamples of the Properties of Logarithms Suppose you are given these two facts: log 4 5=1.16 log 4 10=1.66 Then we can use the laws of logarithms to conclude that: log 4 (50) = log 4 5+log 4 10 = 2.82 log 4 (2) = log 4 10 log 4 5 = 0.5 log 4 (100,000) = 5log 4 10 = 8.3 All three of these results can e found quickly, and without a calculator. Note that the second result could also e figured out directly, since 4 ½ =2. These properties of logarithms were very important historically, ecause they enaled precalculator mathematicians to perform multiplication (which is very time-consuming and error prone) y doing addition (which is faster and easier). These rules are still useful in simplifying complicated epressions and solving equations. Eample: Solving an equation with the properties of logarithms log 2 log 2 (-1)=5 log 2 1 The prolem =5 Second property of logarithms =2 5 =32 Rewrite the log as an eponent. (2-to-what is 1 =32(-1) = = 32 =32/31 Multiply. We now have an easy equation to solve.? 2-to-the-5!) 1 8BProving the Properties of Logarithms If you understand what an eponent is, you can very quickly see why the three rules of eponents work. But why do logarithms have these three properties? As you work through the tet, you will demonstrate these rules intuitively, y viewing the logarithm as a counter. (log 2 8 asks how many 2s do I need to multiply, in order to get 8? ) Chapter 6: Logarithms Conceptual Eplanations 4/8

5 However, these rules can also e rigorously proven, using the laws of eponents as our starting place. Proving the First Law of Logarithms, log (a) = log a+log We appeal to the formal definition of logarithms (three times). log ( a) loga log logalog a( a)( ) ( )( ). Therefore log ( a) log a log, ecause if two powers of the same ase are equal, the eponents must e equal. Done. The key step was when we used the rule for adding eponents. Each of the laws of logarithms corresponds directly to a law of eponents. To test your understanding, prove the second and third laws of logarithms using similar arguments. 9BA Few Important Logarithms The following facts come up often enough that you need to e ale to supply the answers quickly. They are true for any >0: log (1) = 0 log () = 1 log ( 1 a ) = -log a log 0 does not eist These are not special cases! They follow directly from the definition of the logarithm. 6BCommon Logarithms When you see a root without a numer in it, it is assumed to e a square root. That is, 25 is a shorthand way of writing This rule is employed ecause square roots are more common than other types. When you see a logarithm without a numer in it, it is assumed to e a ase 10 logarithm. That is, log(1000) is a shorthand way of writing log 10 (1000). A ase 10 logarithm is also known as a common log. Why are common logs particularly useful? Well, what is log 10 (1000)? By now you know that this asks the question 10 to what power is 1000? The answer is 3. Similarly, you can confirm that: log(10)=1 log(100)=2 log(1,000,000)=6 Chapter 6: Logarithms Conceptual Eplanations 5/8

6 We can also follow this pattern ackward: log(1)=0 log(1/10)=-1 log(1/100)=-2 and so on. In other words, the common log tells you the order of magnitude of a numer: how many zeros it has. Of course, log is difficult to determine eactly without a calculator, ut we can say immediately that it must e somewhere etween 2 and 3, since 500 is etween 100 and BGraphing Logarithmic Functions Suppose you want to graph the function y=log 2 (). You might start y making a tale that looks something like this: y=log um...i m not sure can I use a calculator? This doesn t seem to e the right strategy. Many of those numers are just too hard to work with. So, you start looking for numers that are easy to work with. And you rememer that it s important to look at numers that are less than 1, as well as greater. And eventually, you end up with something more like this. y=log 2 ⅛ 3 ¼ 2 ½ As long as you keep putting powers of 2 in the column, the y column is very easy to figure. In fact, the easiest way to generate this tale is to recognize that it is the tale of y=2 values, only with the and y coordinates switched! In other words, we have re-discovered what we already knew: that y=2 and y=log 2 are inverse functions. When you graph it, you end up with something like this: Chapter 6: Logarithms Conceptual Eplanations 6/8

7 y=log 2 As always, you can learn a great deal aout the log function y reading the graph. The domain is >0. (You can t take the log of 0 or a negative numer do you rememer why?). The range, on the other hand, is all numers. Of course, all this inverses the function 2, which has a domain of all numers and a range of y>0. As gets closer and closer to 0, the function dives down to smaller and smaller negative numers. So the y-ais serves as an asymptote for the graph, meaning a line that the graph approaches closer and closer to as y ecomes more and more negative. As moves to the right, the graph grows ut more and more slowly. As goes from 4 to 8, the graph goes up y 1. As goes from 8 to 16, the graph goes up y another 1. It doesn t make it up another 1 until reaches 32...and so on. This pattern of slower and slower growth is one of the most important characteristics of the log. It can e used to slow down functions that have too wide a range to e practical to work with. Chapter 6: Logarithms Conceptual Eplanations 7/8

8 Eample: Using the log to model a real world prolem Lewis Fry Richardson ( ) was a British meteorologist and mathematician. He was also an active Quaker and committed pacifist, and was one of the first men to apply statistics to the study of human conflict. Richardson catalogued 315 wars etween 1820 and 1950, and categorized them y how many deaths they caused. At one end of the scale is a deadly quarrel, which might result in 1 or 2 deaths. At the other etreme are World War I and World War II, which are responsile for roughly 10 million deaths each. As you can see from the chart aove, working with these numers is etremely difficult: on a scale from 0 to 10 Million, there is no visile difference etween (say) 1 and 100,000. Richardson solved this prolem y taking the common log of the numer of deaths. So a conflict with 1,000 deaths is given a magnitude of log(1000)=3. On this scale, which is now the standard for conflict measurement, the magnitudes of all wars can e easily represented. Richardson s scale makes it practical to chart, discuss, and compare wars and attles from the smallest to the iggest. For instance, he discovered that each time you move up y one on the scale that is, each time the numer of deaths multiplies y 10 the numer of conflicts drops in a third. (So there are roughly three times as many magnitude 5 wars as magnitude 6, and so on.) The log is useful here ecause the logarithm function itself grows so slowly that it compresses the entire 1-to-10,000,000 range into a 0-to-7 scale. As you will see in the tet, the same trick is used for the same reason in fields ranging from earthquakes to sound waves. Chapter 6: Logarithms Conceptual Eplanations 8/8