# 6.4 Logarithmic Equations and Inequalities

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1 6.4 Logarithmic Equations and Inequalities Logarithmic Equations and Inequalities In Section 6.3 we solved equations and inequalities involving exponential functions using one of two basic strategies. We now turn our attention to equations and inequalities involving logarithmic functions, and not surprisingly, there are two basic strategies to choose from. For example, suppose we wish to solve log x) = log 5). Theorem 6.4 tells us that the only solution to this equation is x = 5. Now suppose we wish to solve log x) = 3. If we want to use Theorem 6.4, we need to rewrite 3 as a logarithm base. We can use Theorem 6.3 to do just that: 3 = log 3 ) = log 8). Our equation then becomes log x) = log 8) so that x = 8. However, we could have arrived at the same answer, in fewer steps, by using Theorem 6.3 to rewrite the equation log x) = 3 as 3 = x, or x = 8. We summarize the two common ways to solve log equations below. Steps for Solving an Equation involving Logarithmic Functions. Isolate the logarithmic function.. a) If convenient, express both sides as logs with the same base and equate the arguments of the log functions. b) Otherwise, rewrite the log equation as an exponential equation. Example Solve the following equations. Check your solutions graphically using a calculator.. log 7 3x) = log 7 x 3 ). lnx 3) = 3. log 6 x + 4) + log 6 3 x) = 4. log 7 x) = log 7 3 x) 5. log x + 3) = log 6 x) log 4 x + ) = log x) Solution.. Since we have the same base on both sides of the equation log 7 3x) = log 7 x 3 ), we equate what s inside the logs to get 3x = x 3. Solving x + 3x 4 = 0 gives x = 4 and x =. To check these answers using the calculator, we make use of the change of base formula and graph fx) = ln 3x) ln7) and gx) = lnx 3) ln7) and we see they intersect only at x = 4. To see what happened to the solution x =, we substitute it into our original equation to obtain log 7 ) = log 7 ). While these expressions look identical, neither is a real number, which means x = is not in the domain of the original equation, and is not a solution.. Our first objective in solving lnx 3) = is to isolate the logarithm. We get lnx 3) =, which, as an exponential equation, is e = x 3. We get our solution x = e + 3. On the calculator, we see the graph of fx) = lnx 3) intersects the graph of gx) = at x = e They do, however, represent the same family of complex numbers. We stop ourselves at this point and refer the reader to a good course in Complex Variables.

2 460 Exponential and Logarithmic Functions y = fx) = log 7 3x) and y = fx) = lnx 3) and y = gx) = log 7 x 3 ) y = gx) = 3. We can start solving log 6 x+4)+log 6 3 x) = by using the Product Rule for logarithms to rewrite the equation as log 6 [x + 4)3 x)] =. Rewriting this as an exponential equation, we get 6 = x + 4)3 x). This reduces to x + x 6 = 0, which gives x = 3 and x =. Graphing y = fx) = lnx+4) ln6) x = 3 and x =. + ln3 x) ln6) and y = gx) =, we see they intersect twice, at y = fx) = log 6 x + 4) + log 6 3 x) and y = gx) = 4. Taking a cue from the previous problem, we begin solving log 7 x) = log 7 3 x) by first collecting the logarithms on the same side, log 7 x) + log 7 3 x) =, and then using the Product Rule to get log 7 [ x)3 x)] =. Rewriting this as an exponential equation gives 7 = x)3 x) which gives the quadratic equation x 7x 4 = 0. Solving, we find x = ln x) and x = 4. Graphing, we find y = fx) = ln7) intersect only at x =. Checking x = 4 in the original equation produces log 7 7) = log 7 ), which is a clear domain violation. and y = gx) = ln3 x) ln7) 5. Starting with log x + 3) = log 6 x) + 3, we gather the logarithms to one side and get log x + 3) log 6 x) = 3. We then use the Quotient Rule and convert to an exponential equation ) x + 3 log = 3 3 = x x 6 x This reduces to the linear equation 86 x) = x + 3, which gives us x = 5. When we graph fx) = lnx+3) ln) and gx) = ln6 x) ln) + 3, we find they intersect at x = 5.

3 6.4 Logarithmic Equations and Inequalities 46 y = fx) = log 7 x) and y = fx) = log x + 3) and y = gx) = log 7 3 x) y = gx) = log 6 x) Starting with + log 4 x + ) = log x), we gather the logs to one side to get the equation = log x) log 4 x + ). Before we can combine the logarithms, however, we need a common base. Since 4 is a power of, we use change of base to convert log 4 x + ) = log x + ) log 4) = log x + ) Hence, our original equation becomes = log x) log x + ) ) = log x) log x + ) = log x ) log x + ) Power Rule ) x = log Quotient Rule x + Rewriting this in exponential form, we get formula, we get x = ± 3. Graphing fx) = + lnx+) ln4) x x+ = or x x = 0. Using the quadratic and gx) = lnx) ln), we see the graphs intersect only at x = The solution x = 3 < 0, which means if substituted into the original equation, the term log 3 ) is undefined. y = fx) = + log 4 x + ) and y = gx) = log x)

4 46 Exponential and Logarithmic Functions If nothing else, Example 6.4. demonstrates the importance of checking for extraneous solutions when solving equations involving logarithms. Even though we checked our answers graphically, extraneous solutions are easy to spot - any supposed solution which causes a negative number inside a logarithm needs to be discarded. As with the equations in Example 6.3., much can be learned from checking all of the answers in Example 6.4. analytically. We leave this to the reader and turn our attention to inequalities involving logarithmic functions. Since logarithmic functions are continuous on their domains, we can use sign diagrams. Example Solve the following inequalities. Check your answer graphically using a calculator.. lnx) +. log x)) < log x) x logx + ) x Solution.. We start solving lnx)+ by getting 0 on one side of the inequality: lnx)+ 0. Getting a common denominator yields lnx)+ lnx)+ lnx) lnx)+ 0 which reduces to lnx)+ 0, lnx) lnx)+ lnx)+ or 0. We define rx) = lnx) and set about finding the domain and the zeros of r. Due to the appearance of the term lnx), we require x > 0. In order to keep the denominator away from zero, we solve lnx) + = 0 so lnx) =, so x = e = e. Hence, the domain of r is 0, ) e e, ). To find the zeros of r, we set rx) = lnx) lnx)+ = 0 so that lnx) = 0, and we find x = e 0 =. In order to determine test values for r without resorting to the calculator, we need to find numbers between 0, e, and which have a base of e. Since e.78 >, 0 < < e e < e < < e. To determine the sign of r ) e, we use the fact that ln ) e = ln e ) =, and find r ) e = + =, which is +). The rest of the test values are determined similarly. From our sign diagram, we find the solution to be 0, e) [, ). Graphing fx) = lnx)+ and gx) =, we see the the graph of f is below the graph of g on the solution intervals, and that the graphs intersect at x =. +) ) 0 +) 0 e y = fx) = lnx)+ and y = gx) = Recall that an extraneous solution is an answer obtained analytically which does not satisfy the original equation.

5 6.4 Logarithmic Equations and Inequalities 463. Moving all of the nonzero terms of log x)) < log x) + 3 to one side of the inequality, we have log x)) log x) 3 < 0. Defining rx) = log x)) log x) 3, we get the domain of r is 0, ), due to the presence of the logarithm. To find the zeros of r, we set rx) = log x)) log x) 3 = 0 which results in a quadratic in disguise. We set u = log x) so our equation becomes u u 3 = 0 which gives us u = and u = 3. Since u = log x), we get log x) =, which gives us x = =, and log x) = 3, which yields x = 3 = 8. We use test values which are powers of : 0 < 4 < < < 8 < 6, and from our sign diagram, we see rx) < 0 on, 8). Geometrically, we see the graph of fx) = is below the graph of y = gx) = lnx) ln) + 3 on the solution interval. lnx) ln) ) 0 +) 0 ) 0 +) 8 y = fx) = log x)) and y = gx) = log x) We begin to solve x logx+) x by subtracting x from both sides to get x logx+) x 0. We define rx) = x logx+) x and due to the presence of the logarithm, we require x+ > 0, or x >. To find the zeros of r, we set rx) = x logx + ) x = 0. Factoring, we get x logx + ) ) = 0, which gives x = 0 or logx+) = 0. The latter gives logx+) =, or x + = 0, which admits x = 9. We select test values x so that x + is a power of 0, and we obtain < 0.9 < 0 < 0 < 9 < 99. Our sign diagram gives the solution to be, 0] [9, ). The calculator indicates the graph of y = fx) = x logx + ) is above y = gx) = x on the solution intervals, and the graphs intersect at x = 0 and x = 9. +) 0 ) 0 0 +) 9 y = fx) = x logx + ) and y = gx) = x

6 464 Exponential and Logarithmic Functions Our next example revisits the concept of ph as first introduced in the exercises in Section 6.. Example In order to successfully breed Ippizuti fish the ph of a freshwater tank must be at least 7.8 but can be no more than 8.5. Determine the corresponding range of hydrogen ion concentration, and check your answer using a calculator. Solution. Recall from Exercise 77 in Section 6. that ph = log[h + ] where [H + ] is the hydrogen ion concentration in moles per liter. We require 7.8 log[h + ] 8.5 or 7.8 log[h + ] 8.5. To solve this compound inequality we solve 7.8 log[h + ] and log[h + ] 8.5 and take the intersection of the solution sets. 3 The former inequality yields 0 < [H + ] and the latter yields [H + ] Taking the intersection gives us our final answer [H + ] Your Chemistry professor may want the answer written as [H + ] ) After carefully adjusting the viewing window on the graphing calculator we see that the graph of fx) = logx) lies between the lines y = 7.8 and y = 8.5 on the interval [ , ]. The graphs of y = fx) = logx), y = 7.8 and y = 8.5 We close this section by finding an inverse of a one-to-one function which involves logarithms. Example The function fx) = logx) logx) is one-to-one. Find a formula for f x) and check your answer graphically using your calculator. Solution. We first write y = fx) then interchange the x and y and solve for y. y = fx) y = logx) logx) x = logy) logy) Interchange x and y. x logy)) = logy) x x logy) = logy) x = x logy) + logy) x = x + ) logy) x = logy) x + y = 0 x x+ Rewrite as an exponential equation. 3 Refer to page 4 for a discussion of what this means.

7 6.4 Logarithmic Equations and Inequalities 465 We have f x) = 0 x x+. Graphing f and f on the same viewing window yields y = fx) = logx) logx) and y = gx) = 0 x x+

8 466 Exponential and Logarithmic Functions 6.4. Exercises In Exercises - 4, solve the equation analytically.. log3x ) = log4 x). log x 3 ) = log x) 3. ln 8 x ) = ln x) 4. log 5 8 x ) = log 5 6 x) 5. log 3 7 x) = 6. log x ) = 3 7. ln x 99 ) = 0 8. logx 3x) = ) 3x 9. log 5 = x logx) = log x ) 0. log 0 3 = 4.7 x 0 ) = log 5 x) = lnx) = lnx) 5. log 3 x 4) + log 3 x + 4) = 6. log 5 x + ) + log 5 x + ) = 7. log 69 3x + 7) log 69 5x 9) = 8. lnx + ) lnx) = 3 9. log 7 x) = log 7 ) + log 7 x + ) 0. logx) log) = logx + 8) logx + ). log 3 x) = log x) + 8. lnlnx)) = logx)) = logx) lnx ) = lnx)) In Exercises 5-30, solve the inequality analytically. lnx) 5. x < 0 6. x lnx) x > 0 x ) x ) 7. 0 log log < logx) < lnx ) lnx)) In Exercises 3-34, use your calculator to help you solve the equation or inequality. 3. lnx) = e x 3. lnx) = 4 x 33. lnx + ) ln x 3 x + 3x 6) < 0

9 6.4 Logarithmic Equations and Inequalities Since fx) = e x is a strictly increasing function, if a < b then e a < e b. Use this fact to solve the inequality lnx + ) < 3 without a sign diagram. Use this technique to solve the inequalities in Exercises 7-9. Compare this to Exercise 46 in Section 6.3.) 36. Solve ln3 y) lny) = x + ln5) for y. 37. In Example we found the inverse of fx) = logx) logx) to be f x) = 0 x x+. a) Show that f f ) x) = x for all x in the domain of f and that f f ) x) = x for all x in the domain of f. b) Find the range of f by finding the domain of f. c) Let gx) = x x and hx) = logx). Show that f = g h and g h) = h g. We know this is true in general by Exercise 3 in Section 5., but it s nice to see a specific example of the property.) 38. Let fx) = ) + x ln. Compute f x) and find its domain and range. x 39. Explain the equation in Exercise 0 and the inequality in Exercise 8 above in terms of the Richter scale for earthquake magnitude. See Exercise 75 in Section 6..) 40. Explain the equation in Exercise and the inequality in Exercise 7 above in terms of sound intensity level as measured in decibels. See Exercise 76 in Section 6..) 4. Explain the equation in Exercise and the inequality in Exercise 9 above in terms of the ph of a solution. See Exercise 77 in Section 6..) 4. With the help of your classmates, solve the inequality n x > lnx) for a variety of natural numbers n. What might you conjecture about the speed at which fx) = lnx) grows versus any principal n th root function?

10 468 Exponential and Logarithmic Functions 6.4. Answers. x = 5 4. x = 3. x = 4. x = 3, 4 5. x = 6. x = 9 7. x = ±0 8. x =, 5 9. x = x = 0.7. x = x = x = 5 4. x = e 3/4 5. x = 5 6. x = 7. x = 8. x = e 3 9. x = 6 0. x = 4. x = 8. x = e e3 3. x = 0 3, x =, x = e 5. e, ) 6. e, ) 7. [ 0 3, ) 8. [ 0.6, 0 4.] , 0.3) 30. 0, ] [e, ) 3. x x 4.77, x ,.44).44, ) , 3) 0.5, 0.599).999, ) 35. < x < e3 36. y = 3 5e x f x) = ex e x + = ex e x e x. To see why we rewrite this in this form, see Exercise 5 in + e x Section.0.) The domain of f is, ) and its range is the same as the domain of f, namely, ).

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