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Find the average value of the function 
f(x)=(3)/(9-4x) from 
x=3 to 
x=6. Express your answer as a constant times 
ln 5.
Answer: 
◻ln 5

Find the average value of the function f(x)=394x f(x)=\frac{3}{9-4 x} from x=3 x=3 to x=6 x=6 . Express your answer as a constant times ln5 \ln 5 .\newlineAnswer: ln5 \square\ln 5

Full solution

Q. Find the average value of the function f(x)=394x f(x)=\frac{3}{9-4 x} from x=3 x=3 to x=6 x=6 . Express your answer as a constant times ln5 \ln 5 .\newlineAnswer: ln5 \square\ln 5
  1. Understand average value concept: Understand the concept of average value of a function. The average value of a function f(x)f(x) on the interval [a,b][a, b] is given by the formula: Average value = 1(ba)abf(x)dx\frac{1}{(b-a)} \int_{a}^{b} f(x) \, dx Here, a=3a = 3 and b=6b = 6, so we need to integrate f(x)=3(94x)f(x) = \frac{3}{(9-4x)} from x=3x = 3 to x=6x = 6 and then multiply by the reciprocal of the interval's length.
  2. Set up integral for average value: Set up the integral for the average value.\newlineAverage value = (1/(63))×363(94x)dx(1/(6-3)) \times \int_{3}^{6} \frac{3}{(9-4x)} \, dx\newlineThis simplifies to:\newlineAverage value = (1/3)×363(94x)dx(1/3) \times \int_{3}^{6} \frac{3}{(9-4x)} \, dx
  3. Perform substitution to simplify integral: Perform a substitution to simplify the integral.\newlineLet u=94xu = 9 - 4x, then du=4dxdu = -4 dx or dx=du4dx = -\frac{du}{4}.\newlineWhen x=3x = 3, u=94(3)=912=3u = 9 - 4(3) = 9 - 12 = -3.\newlineWhen x=6x = 6, u=94(6)=924=15u = 9 - 4(6) = 9 - 24 = -15.\newlineNow we can rewrite the integral in terms of uu:\newlineAverage value = (1/3)×315(34)×(1u)du(1/3) \times \int_{-3}^{-15} (-\frac{3}{4}) \times (\frac{1}{u}) du
  4. Evaluate the integral: Evaluate the integral.\newlineThe integral of 1u\frac{1}{u} with respect to uu is lnu\ln|u|, so we have:\newlineAverage value = (13)(34)[lnu](\frac{1}{3}) \cdot (\frac{-3}{4}) \cdot [\ln|u|] from 3-3 to 15-15\newlineThis simplifies to:\newlineAverage value = (14)[ln15ln3](-\frac{1}{4}) \cdot [\ln|-15| - \ln|-3|]\newlineSince lnalnb=lnab\ln|a| - \ln|b| = \ln|\frac{a}{b}|, we can combine the logarithms:\newlineAverage value = (14)ln(153)(-\frac{1}{4}) \cdot \ln(\frac{15}{3})\newlineAverage value = (14)ln(5)(-\frac{1}{4}) \cdot \ln(5)
  5. Check for errors and write final answer: Check for math errors and write the final answer.\newlineWe have correctly applied the rules of logarithms and the integral was evaluated correctly. The negative sign comes from the fact that dudu was 4dx-4 dx, which we accounted for in the integral.\newlineFinal answer: (1/4)ln(5)(-1/4)\ln(5)