Chabot Mathematics §4.2 Log Functions Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu Chabot College Mathematics 1 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Review § 4.1 Any QUESTIONS About • §4.1 → Exponential Functions Any QUESTIONS About HomeWork • §4.1 → HW-18 Chabot College Mathematics 2 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx §4.2 Learning Goals Define and explore logarithmic functions and their properties Use logarithms to solve exponential equations Examine applications involving logarithms John Napier (1550-1617) • Logarithm Pioneer Chabot College Mathematics 3 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Logarithm → What is it? Concept: If b > 0 and b ≠ 1, then y = logbx is equivalent to x = by Symbolically The exponent is the logarithm. x = by y = logbx The base is the base of the logarithm. Chabot College Mathematics 4 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Logarithm Illustrated Consider the exponential function y = f(x) = 3x. Like all exponential functions, f is one-to-one. Can a formula for the inverse Function, x = g(y) be found? x = 3y y = 3x Need y ≡ the exponent to which we must raise 3 to get x. f −1(x) ≡ the exponent to which we must raise 3 to get x. Chabot College Mathematics 5 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Logarithm Illustrated Now define a new symbol to replace the words “the exponent to which we must raise 3 to get x”: log3x, read “the logarithm, base 3, of x,” or “log, base 3, of x,” means “the exponent to which we raise 3 to get x.” Thus if f(x) = 3x, then f−1(x) = log3x. Note that f−1(9) = log39 = 2, as 2 is the exponent to which we raise 3 to get 9 Chabot College Mathematics 6 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Evaluate Logarithms Evaluate: a) log381 b) log31 c) log3(1/9) Solution: a) Think of log381 as the exponent to which we raise 3 to get 81. The exponent is 4. Thus, since 34 = 81, log381 = 4. b) ask: “To what exponent do we raise 3 in order to get 1?” That exponent is 0. So, log31 = 0 c) To what exponent do we raise 3 in order to get 1/9? Since 3−2 = 1/9 we have log3(1/9) = −2 Chabot College Mathematics 7 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx The Meaning of logax For x > 0 and a a positive constant other than 1, logax is the exponent to which a must be raised in order to get x. Thus, logax = m means am = x or equivalently, logax is that unique exponent for which Chabot College Mathematics 8 a loga x x Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Exponential to Log Write each exponential equation in logarithmic form. 4 2 1 1 3 c. a 7 a. 4 64 b. 2 16 3 Soln a. 4 64 log 64 3 4 4 1 1 1 b. log1 2 4 2 16 16 c. a 2 7 log a 7 2 Chabot College Mathematics 9 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Log to Exponential Write each logarithmic equation in exponential form a. log 3 243 5 b. log 2 5 x c. log a N x Soln a. log 3 243 5 243 35 b. log 2 5 x 5 2 x c. log a N x N a Chabot College Mathematics 10 x Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Evaluate Logarithms Find the value of each of the following logarithms c. log1 3 9 a. log 5 25 b. log 2 16 1 d. log 7 7 e. log 6 1 f. log 4 2 Solution a. log 5 25 y 25 5 or 5 5 y y2 b. log 2 16 y 16 2 or 2 2 y y4 y y Chabot College Mathematics 11 2 4 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Evaluate Logarithms Solution (cont.) y 1 2 y c. log1 3 9 y 9 or 3 3 3 d. log 7 7 y 7 7 y or 71 7 y e. log 6 1 y 1 6 or 6 6 y 0 y y 1 y0 1 1 y 1 2y f. log 4 y 4 or 2 2 2 2 Chabot College Mathematics 12 y 2 1 y 2 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Use Log Definition Solve each equation for x, y or z 1 a. log 5 x 3 b. log 3 y 27 2 d. log 2 x 6x 10 1 c. log z 1000 3 Solution a. log 5 x 3 3 x5 1 1 x 3 5 125 Chabot College Mathematics 13 1 The solution set is . 125 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Use Log Definition Solution (cont.) 1 b. log 3 y 27 1 3y 27 33 3y 3 y c. log z 1000 3 1000 z 3 10 z 10 z The solution set is 10. 3 3 The solution set is 3. Chabot College Mathematics 14 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Inverse Property of Logarithms Recall Def: For x > 0, a > 0, and a ≠ 1, y log a x if and only if xa . y In other words, The logarithmic function is the inverse function of the exponential function; e.g. log a a x x Chabot College Mathematics 15 a log a x x Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Derive Change of Base Rule Any number >1 can be used for b, but since most calculators have ln and log functions we usually change between base-e and base-10 Chabot College Mathematics 16 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Inverse Property Evaluate: log 5 23 5 . Solution Remember that log523 is the exponent to which 5 is raised to get 23. Raising 5 to that exponent, obtain log 5 23 5 Chabot College Mathematics 17 23. Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Basic Properties of Logarithms For any base a > 0, with a ≠ 1, Discern from the Log Definition 1. Logaa = 1 • As 1 is the exponent to which a must be raised to obtain a (a1 = a) 2. Loga1 = 0 • As 0 is the exponent to which a must be raised to obtain 1 (a0 = 1) Chabot College Mathematics 18 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Graph Logarithmic Function Sketch the graph of y = log3x x y = log3x (x, y) –3 (1/27, –3) –2 (1/9, –2) –1 (1/3, –1) 30 = 1 0 (1, 0) 31 = 3 1 (3, 1) 32 = 9 2 (9, 2) Soln: 3–3 = 1/27 Make 3–2 = 1/9 T-Table –3 = 1/3 3 → Chabot College Mathematics 19 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Graph Logarithmic Function Plot the ordered pairs and connect the dots with a smooth curve to obtain the graph of y = log3x Chabot College Mathematics 20 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Graph by Inverse Graph y = f(x) = 3x Solution: Use Inverse Relation for Logs & Exponentials Reflect the graph of y = 3x across the line y = x to obtain the graph of y = f−1(x) = log3x Chabot College Mathematics 21 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Properties of Exponential and Logarithmic Functions Exponential Function f (x) = ax Logarithmic Function f (x) = loga x Domain (–∞, ∞) Range (0, ∞) Domain (0, ∞) Range (–∞, ∞) y-intercept is 1 No x-intercept x-intercept is 1 No y-intercept x-axis (y = 0) is the horizontal asymptote y-axis (x = 0) is the vertical asymptote Chabot College Mathematics 22 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Properties of Exponential and Logarithmic Functions Exponential Function f (x) = ax Logarithmic Function f (x) = loga x Is one-to-one , that is, au = av if and only if u = v Is one-to-one, that is, logau = logav if and only if u = v Increasing if a > 1 Decreasing if 0 < a < 1 Increasing if a > 1 Decreasing if 0 < a < 1 Chabot College Mathematics 23 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Graphs of Logarithmic Fcns f (x) = loga x (a > 1) Chabot College Mathematics 24 f (x) = loga x (0 < a < 1) Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Common Logarithms The logarithm with base 10 is called the common logarithm and is denoted by omitting the base: logx = log10x. So y = logx if and only if x = 10y Applying the basic properties of logs 1. log(10) = 1 2. log(1) = 0 3. log(10x) = x 4. 10logx = x Chabot College Mathematics 25 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Common Log Convention By this Mathematics CONVENTION the abbreviation log, with no base written, is understood to mean logarithm base 10, or a common logarithm. Thus, log21 = log1021 On most calculators, the key for LOG common logarithms is marked Chabot College Mathematics 26 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Natural Logarithms Logarithms to the base “e” are called natural logarithms, or Napierian logarithms, in honor of John Napier, who first “discovered” logarithms. The abbreviation “ln” is generally used with natural logarithms. Thus, ln 21 = loge 21. On most calculators, the key for natural logarithms is marked LN Chabot College Mathematics 27 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Natural Logarithms The logarithm with base e is called the natural logarithm and is denoted by ln x. That is, ln x = loge x. So y = lnx if and only if x = ey Applying the basic properties of logs 1. ln(e) = 1 2. ln(1) = 0 3. ln(ex) = x 4. elnx = x Chabot College Mathematics 28 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Sound Intensity This function is sometimes I used to calculate d 10log sound intensity I0 Where • d ≡ the intensity in decibels, • I ≡ the intensity watts per unit of area • I0 ≡ the faintest audible sound to the average human ear, which is 10−12 watts per square meter (1x10−12 W/m2). Chabot College Mathematics 29 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Sound Intensity Use the Sound Intensity Equation (a.k.a. the “dBA” Eqn) to find the intensity level of sounds at a decibel level of 75 dB? Solution: We need to isolate the intensity, I, in the dBA eqn Chabot College Mathematics 30 I d 10log , I0 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Sound Intensity Solution (cont.) in the dBA eqn substitute 75 for d and 10−12 for I0 and then solve for I I 75 10 log 12 10 I 1012 I 1012 107.5 7.5 log 12 12 10 10 I 4.5 7.5 10 I 10 10 12 Chabot College Mathematics 31 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Sound Intensity Thus the Sound Intensity at 75 dB is 10−4.5 W/m2 = 10−9/2 W/m2 Using a Scientific calculator and find that I = 3.162x10−5 W/m2 = 31.6 µW/m2 Chabot College Mathematics 32 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Sound Intensity Check If the sound intensity is 10−4.5 W/m2 , verify that the decibel reading is 75. 104.5 d 10log 1012 7.5 d 10log10 d 10 7.5 Chabot College Mathematics 33 d 75 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Summary of Log Rules For any positive numbers M, N, and a with a ≠ 1, and whole number p log a ( MN ) log a M log a N ; log a M p p log a M ; Power Rule M log a log a M log a N ; N k log a a k . Chabot College Mathematics 34 Product Rule Quotient Rule Base-to-Power Rule Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Typical Log-Confusion Beware that Logs do NOT behave Algebraically. In General: log a ( MN ) (log a M )(log a N ), M log a M log a , N log a N log a ( M N ) log a M log a N , log a ( M N ) log a M log a N . Chabot College Mathematics 35 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Change of Base Rule Let a, b, and c be positive real numbers with a ≠ 1 and b ≠ 1. Then logbx can be converted to a different base as follows: log a x log x ln x log b x log a b log b ln b (base a) (base 10) (base e) Chabot College Mathematics 36 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Derive Change of Base Rule Any number >1 can be used for b, but since most calculators have ln and log functions we usually change between base-e and base-10 Chabot College Mathematics 37 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Evaluate Logs Compute log513 by changing to (a) common logarithms (b) natural logarithms Soln Chabot College Mathematics 38 log13 a. log 5 13 log 5 1.59369 ln13 b. log 5 13 ln 5 1.59369 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Evaluate Logs Use the change-of-base formula to calculate log512. • Round the answer to four decimal places Solution Check Chabot College Mathematics 39 log12 log 5 12 log5 1.5440 51.5440 12.0009 12 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Evaluate Logs Find log37 using the change-of-base formula log10 7 Solution log3 7 log10 3 0.84509804 0.47712125 Substituting into log a M logb M . log a b 1.7712 3 7.000 1.7712 Chabot College Mathematics 40 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Use The Rules Express as an equivalent expression using individual logarithms of x, y, & z x3 xy a) log 4 b) logb 3 yz z7 3 x 3 – log yz a) log = log x 4 Soln 4 4 yz a) = 3log4x – log4 yz = 3log4x – (log4 y + log4z) = 3log4x – log4 y – log4z Chabot College Mathematics 41 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Use The Rules 1/ 3 Soln xy xy 3 b) log logb b b) 7 7 z z 1 xy logb 7 3 z 1 logb xy logb z 7 3 1 logb x logb y 7logb z 3 Chabot College Mathematics 42 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Caveat on Log Rules Because the product and quotient rules replace one term with two, it is often best to use the rules within parentheses, as in the previous example 1 xy logb 3 z7 1 logb x logb y 7logb z 3 Chabot College Mathematics 43 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Cesium-137 ½-Life A sample of radioactive Cesium-137 has been Stored, unused, for cancer treatment for 2.2 years. In that time, 5% of the original sample has decayed. What is the half-life (time required to reduce the radioactive substance to half of its starting quantity) of Cesium-137? Chabot College Mathematics 44 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Cesium-137 ½-Life SOLUTION: Start with the math model kt A t A0 e for exponential Decay Recall the Given information: after 2.2 years, 95% of the sample remains Use the Model and given data to find k Use data in Model: 0.95 A0 A0e k 2.2 Divide both sides by A0: 0.95 e 2.2 k Chabot College Mathematics 45 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Cesium-137 ½-Life Now take the ln of both Sides ln 0.95 e 2.2 k ln 0.95 ln e 2.2 k Using the Base-to-Power Rule ln 0.95 ln e 2.2 k log e e 2.2 k ln 0.95 2.2k Find by Algebra Now set the amount, A, to ½ of A0 A0 At HL A0 e 0.0233t HL 2 Chabot College Mathematics 46 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Cesium-137 ½-Life After dividing both 0.5 e 0.0233t sides by A0 Taking the ln of Both Sides ln 0.5 ln e 0.0233t HL HL 0.6931 0.0233t HL Solving for 0.6931 t HL 29.73 the HalfLife 0.0233 State: The HalfLife of Cesion-137 is approximately 29.7 years Chabot College Mathematics 47 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Compound Interest In a Bank Account that Compounds CONTINUOUSLY the relationship between the $-Principal, P, deposited, the Interest rate, r, the Compounding time-period, t, and the $-Amount, A, in the Account: 1 A t ln r P Chabot College Mathematics 48 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Compound Interest If an account pays 8% annual interest, compounded continuously, how long will it take a deposit of $25,000 to produce an account balance of $100,000? Familiarize In the Compounding Eqn replace P with 25,000, r with 0.08, A with $100,000, and then simplify. Chabot College Mathematics 49 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Compound Interest Solution 1 100, 000 t ln 0.08 25, 000 1 t ln 4 0.08 t 17.33 Substitute. Divide. Approximate using a calculator. State Answer The account balance will reach $100,000 in about 17.33 years. Chabot College Mathematics 50 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Example Compound Interest Check: 1 A 17.33 ln 0.08 25, 000 A 1.3864 ln 25, 000 1.3864 ln A ln 25, 000 1.3864 ln 25, 000 ln A 11.513 ln A e11.513 A 100, 007.5 A Because 17.33 was not the exact time, $100,007.45 is reasonable for the Chk Chabot College Mathematics 51 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx WhiteBoard Work Problems From §4.2 • P72 → Atmospheric Pressure at Altitude – See also: B. Mayer, “Small Signal Analysis of Source Vapor Control Requirements for APCVD”, IEEE Transactions on Semiconductor Manufacturing, vol. 9, no. 3, 1996, pg 355 Chabot College Mathematics 52 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx All Done for Today Napier’s MasterWork Year 1619 Chabot College Mathematics 53 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Chabot Mathematics Appendix r s r s r s 2 2 Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu – Chabot College Mathematics 54 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx ConCavity Sign Chart ConCavity Form d2f/dx2 Sign ++++++ Critical (Break) Points Chabot College Mathematics 55 −−−−−− a Inflection −−−−−− b NO Inflection ++++++ c Inflection Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx x Chabot College Mathematics 56 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Chabot College Mathematics 57 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx Chabot College Mathematics 58 Bruce Mayer, PE BMayer@ChabotCollege.edu • MTH15_Lec-19_sec_4-2_Logarithmic_Fcns.pptx