Logarithms and logarithmic functions

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Transcript of Logarithms and logarithmic functions

  • 1. Introduction To Logarithms

2. Why is logarithmic scale used to measure sound? 3. Our first question then must be: What is a logarithm ? 4. One one Function 5. (3,8) (2,4) (1,2) (-1,1/2) (-2,1/4) (8,3) (4,2) (2,1) (1/2,-1) (-1/4,-2) Inverse of 6. The inverse ofis the function 7. (3,8) (2,4) (1,2) (-1,1/2) (-2,1/4) 8. Domain of logarithmic function = Range of exponential function = Range of logarithmic function = Domain of exponential function = 9. 1.Thex -intercept of the graph is 1.There is noy -intercept. 2.They -axis is a vertical asymptote of the graph. 3.A logarithmic function is decreasing if0 1. 4.The graph contains the points (1,0) and (a,1). Properties of the Graph of a Logarithmic Function 10. Logarithmic Abbreviations

  • log 10x = logx(Common log)
  • log ex = ln x(Natural log)
  • e = 2.71828...

11. Of course logarithms havea precise mathematicaldefinition just like all terms in mathematics. So lets start with that. 12. Definition of Logarithm Suppose b>0 and b1,there is a number psuch that: 13. 14. The first, and perhaps the most important step, in understanding logarithms is to realize that they always relate back to exponential equations. 15. You must be able to convert an exponential equation into logarithmic form and vice versa. So lets get a lot of practice with this ! 16. Example 1: Solution: We read this as: the log base 2 of 8 is equal to 3. 17. Example 1a: Solution: Read as: the log base 4 of 16 is equal to 2. 18. Example 1b: Solution: 19. Okay, so now its time for you to try some on your own. 20. Solution: 21. Solution: 22. Solution: 23. It is also very important to be able to start with a logarithmic expression and change this into exponential form. This is simply the reverse ofwhat we just did. 24. Example 1: Solution: 25. Example 2: Solution: 26. Okay, now you try these next three. 27. Solution: 28. Solution: 29. Solution: 30. We now know that a logarithm is perhaps best understoodas being closely related to an exponential equation. In fact, whenever we get stuck in the problems that follow we will return to this one simple insight. We might even state a simple rule. 31. When working with logarithms, if ever you get stuck, try rewriting the problem in exponential form. Conversely, when working with exponential expressions, if ever you get stuck, try rewriting the problem in logarithmic form. 32. Lets see if this simple rule can help us solve some of the following problems. 33. Solution: Lets rewrite the problem in exponential form. Were finished ! 34. Solution: Rewrite the problem in exponential form. 35. Example 3 Try setting this up like this: Solution: Now rewrite in exponential form. 36. These next two problems tend to be some of the trickiest to evaluate. Actually, they are merely identities andthe use of our simple rulewill show this. 37. Example 4 Solution: Now take it out of the logarithmic formand write it in exponential form. First, we write the problem with a variable. 38. Example 5 Solution: First, we write the problem with a variable. Now take it out of the exponential formand write it in logarithmic form. 39. Ask your teacher about the last two examples.They may show you a nice shortcut. 40. Finally, we want to take a look at the Property of Equality for Logarithmic Functions. Basically, with logarithmic functions, if the bases match on both sides of the equal sign , then simply set the arguments equal. 41. Logarithmic Equations 42. Example 1 Solution: Since the bases are both 3 we simply set the arguments equal. 43. Example 2 Solution: Since the bases are both 8 we simply set the arguments equal. Factor continued on the next page 44. Example 2 continued Solution: It appears that we have 2 solutions here. If we take a closer look at the definition of a logarithm however, we will see that not only must we use positive bases, but also we see that the arguments must be positive as well. Therefore -2 is not a solution. Lets end this lesson by taking a closer look at this. 45. Our final concernthen is to determine why logarithms like the one below are undefined. Can anyone give us an explanation ? 46. One easy explanation is to simply rewrite this logarithm in exponential form.Well then see why a negative value is not permitted. First, we write the problem with a variable. Now take it out of the logarithmic formand write it in exponential form. What power of 2 would gives us -8 ? Hence expressions of this type are undefined. 47. 48. 49. That concludes our introduction to logarithms. In the lessons to follow we will learn some important properties of logarithms. One of these properties will give us a very important toolwhich we need to solve exponential equations. Until then lets practice with the basic themes of this lesson. 50. Thats All Folks !