Math Problem Statement

Let f(x) = 1/(x-1)

. Calculate the difference quotient f(-1+h) - f(-1)/ h

for h= .1 h= .01 h= -0.1 h= -.1

If someone now told you that the derivative (slope of the tangent line to the graph) of f(x) at x=-1 was - (1/n^2)

for some positive integer n what would you expect n to be?

Solution

We will calculate the difference quotient:

f(1+h)f(1)h\frac{f(-1+h) - f(-1)}{h}

for the function f(x)=1x1f(x) = \frac{1}{x-1} and the specified values of hh. Then, we will infer the integer nn such that the derivative matches 1n2-\frac{1}{n^2}.


Step 1: Evaluate f(1)f(-1)

f(1)=111=12=12.f(-1) = \frac{1}{-1 - 1} = \frac{1}{-2} = -\frac{1}{2}.

Step 2: General form of f(1+h)f(-1 + h)

f(1+h)=1(1+h)1=12+h.f(-1+h) = \frac{1}{(-1 + h) - 1} = \frac{1}{-2 + h}.

Step 3: Difference Quotient

f(1+h)f(1)h=12+h(12)h=12+h+12h.\frac{f(-1+h) - f(-1)}{h} = \frac{\frac{1}{-2+h} - \left(-\frac{1}{2}\right)}{h} = \frac{\frac{1}{-2+h} + \frac{1}{2}}{h}.

Simplify: f(1+h)f(1)h=12+h+12h=2+(2+h)(2+h)(2)h=h(2+h)(2)h.\frac{f(-1+h) - f(-1)}{h} = \frac{\frac{1}{-2+h} + \frac{1}{2}}{h} = \frac{\frac{2 + (-2 + h)}{(-2+h)(-2)}}{h} = \frac{\frac{h}{(-2+h)(-2)}}{h}.

Cancel hh in the numerator and denominator: f(1+h)f(1)h=1(2+h)(2).\frac{f(-1+h) - f(-1)}{h} = \frac{1}{(-2+h)(-2)}.


Step 4: Compute for Specific Values of hh

  1. h=0.1h = 0.1: 1(2+0.1)(2)=1(1.9)(2)=13.80.2632.\frac{1}{(-2 + 0.1)(-2)} = \frac{1}{(-1.9)(-2)} = \frac{1}{3.8} \approx 0.2632.

  2. h=0.01h = 0.01: 1(2+0.01)(2)=1(1.99)(2)=13.980.2513.\frac{1}{(-2 + 0.01)(-2)} = \frac{1}{(-1.99)(-2)} = \frac{1}{3.98} \approx 0.2513.

  3. h=0.1h = -0.1: 1(20.1)(2)=1(2.1)(2)=14.20.2381.\frac{1}{(-2 - 0.1)(-2)} = \frac{1}{(-2.1)(-2)} = \frac{1}{4.2} \approx 0.2381.

  4. h=0.01h = -0.01: 1(20.01)(2)=1(2.01)(2)=14.020.2488.\frac{1}{(-2 - 0.01)(-2)} = \frac{1}{(-2.01)(-2)} = \frac{1}{4.02} \approx 0.2488.


Step 5: Derivative and Value of nn

The derivative of f(x)=1x1f(x) = \frac{1}{x-1} at x=1x = -1 is computed as: f(x)=1(x1)2.f'(x) = -\frac{1}{(x-1)^2}.

At x=1x = -1: f(1)=1(2)2=14.f'(-1) = -\frac{1}{(-2)^2} = -\frac{1}{4}.

Given the form 1n2-\frac{1}{n^2}, it matches when n=2n = 2.


Final Answers:

  1. f(1+h)f(1)h\frac{f(-1+h) - f(-1)}{h} for:

    • h=0.1h = 0.1: 0.26320.2632
    • h=0.01h = 0.01: 0.25130.2513
    • h=0.1h = -0.1: 0.23810.2381
    • h=0.01h = -0.01: 0.24880.2488
  2. n=2n = 2.


Would you like me to expand on any of these steps? Here are 5 questions for further exploration:

  1. How does the difference quotient approach the derivative as h0h \to 0?
  2. Why does the derivative have the form 1n2-\frac{1}{n^2}?
  3. How would this analysis change for a different function like f(x)=1x+1f(x) = \frac{1}{x+1}?
  4. Can we visualize these calculations on a graph of f(x)=1x1f(x) = \frac{1}{x-1}?
  5. How do the values of hh influence the accuracy of the derivative approximation?

Tip: Always verify difference quotient computations by simplifying the algebraic expression before substituting values for hh.

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Math Problem Analysis

Mathematical Concepts

Algebra
Calculus
Difference Quotient
Derivatives

Formulas

Difference quotient: (f(x+h) - f(x)) / h
Derivative of f(x) = 1/(x-1): f'(x) = -1/(x-1)^2

Theorems

Limit definition of a derivative

Suitable Grade Level

Grades 11-12 or College Introductory Calculus