Math Problem Statement

A logarithmic function of the form y, equals, a, plus, b, natural log, left parenthesis, x, minus, 5, right parenthesisy=a+bln(x−5) passes through the points left parenthesis, 18, comma, 17, right parenthesis(18,17) and left parenthesis, 930, comma, minus, 16, right parenthesis(930,−16). Find a,a and bb, rounding all values to three decimal places, if necessary.

Solution

The problem is to find the constants aa and bb in the logarithmic function:

y=a+bln(x5)y = a + b \ln(x - 5)

where the function passes through the points (18,17)(18, 17) and (930,16)(930, -16).

We can set up a system of equations using these two points. For each point, we substitute the corresponding values of xx and yy into the equation.

Step 1: Write equations for both points

For the point (18,17)(18, 17): 17=a+bln(185)17 = a + b \ln(18 - 5) 17=a+bln(13)17 = a + b \ln(13)

For the point (930,16)(930, -16): 16=a+bln(9305)-16 = a + b \ln(930 - 5) 16=a+bln(925)-16 = a + b \ln(925)

Now we have the system of two equations: 17=a+bln(13)17 = a + b \ln(13) 16=a+bln(925)-16 = a + b \ln(925)

Step 2: Solve the system of equations

Subtract the first equation from the second:

(16)17=(a+bln(925))(a+bln(13))(-16) - 17 = \left(a + b \ln(925)\right) - \left(a + b \ln(13)\right) 33=b(ln(925)ln(13))-33 = b \left(\ln(925) - \ln(13)\right) 33=bln(92513)-33 = b \ln\left(\frac{925}{13}\right)

We calculate: 9251371.1538\frac{925}{13} \approx 71.1538 ln(71.1538)4.266\ln(71.1538) \approx 4.266

Thus, the equation becomes: 33=b×4.266-33 = b \times 4.266

Solve for bb: b=334.2667.741b = \frac{-33}{4.266} \approx -7.741

Step 3: Solve for aa

Now substitute b=7.741b = -7.741 into the first equation: 17=a+(7.741)ln(13)17 = a + (-7.741) \ln(13)

We know that ln(13)2.5657\ln(13) \approx 2.5657, so: 17=a7.741×2.565717 = a - 7.741 \times 2.5657 17=a19.87717 = a - 19.877

Solve for aa: a=17+19.87736.877a = 17 + 19.877 \approx 36.877

Final Answer:

The values of aa and bb are: a36.877andb7.741a \approx 36.877 \quad \text{and} \quad b \approx -7.741

Would you like further explanation on any part of the solution or have any questions? Here are some related questions that might interest you:

  1. How does the natural logarithm function behave with large values of xx?
  2. What happens if the function were to pass through more than two points?
  3. How can you graph a logarithmic function with the parameters found?
  4. What is the effect of changing the constant aa in the function?
  5. How would you find the constants aa and bb for an exponential function instead?

Tip: When working with logarithmic functions, always check if the argument inside the logarithm is positive since the logarithm of a non-positive number is undefined.

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Math Problem Analysis

Mathematical Concepts

Logarithmic Functions
Systems of Equations
Natural Logarithms

Formulas

y = a + b ln(x - 5)

Theorems

Logarithmic properties
Solving systems of linear equations

Suitable Grade Level

Grades 10-12