Math Problem Statement

Evaluate lim x→8 3 √x−2 x−8 .

Solution

We are tasked with evaluating the limit:

limx8x32x8\lim_{x \to 8} \frac{\sqrt[3]{x} - 2}{x - 8}

Direct substitution of x=8x = 8 results in an indeterminate form 00\frac{0}{0}, so we must use algebraic techniques to simplify the expression. In this case, we can use the technique of rationalizing the numerator, as the cube root function is involved.

Step 1: Rewrite the expression

We need to simplify the given limit:

x32x8\frac{\sqrt[3]{x} - 2}{x - 8}

To rationalize, we multiply both the numerator and the denominator by the expression that will eliminate the cube root in the numerator. This is:

[ \frac{(\sqrt[3]{x} - 2)(\sqrt[3]{x^2} + 2\sqrt[3]{x} + 4)}{(x - 8)(\sqrt[3]{x^2} + 2\sqrt[3]{x} + 4)} ]

Step 2: Simplify using the identity

Using the identity (ab)(a2+ab+b2)=a3b3(a - b)(a^2 + ab + b^2) = a^3 - b^3, where a=x3a = \sqrt[3]{x} and b=2b = 2, we get:

[ (\sqrt[3]{x} - 2)(\sqrt[3]{x^2} + 2\sqrt[3]{x} + 4) = x - 8 ]

Thus, the expression becomes:

[ \frac{x - 8}{(x - 8)(\sqrt[3]{x^2} + 2\sqrt[3]{x} + 4)} = \frac{1}{\sqrt[3]{x^2} + 2\sqrt[3]{x} + 4} ]

Step 3: Evaluate the limit

Now, we substitute x=8x = 8 into the simplified expression:

[ \frac{1}{\sqrt[3]{8^2} + 2\sqrt[3]{8} + 4} = \frac{1}{\sqrt[3]{64} + 2 \cdot \sqrt[3]{8} + 4} ]

Since 643=4\sqrt[3]{64} = 4 and 83=2\sqrt[3]{8} = 2, we have:

14+22+4=14+4+4=112\frac{1}{4 + 2 \cdot 2 + 4} = \frac{1}{4 + 4 + 4} = \frac{1}{12}

Final Answer:

limx8x32x8=112\lim_{x \to 8} \frac{\sqrt[3]{x} - 2}{x - 8} = \frac{1}{12}

Would you like further explanation or details on any step? Here are some related questions you can explore:

  1. How can you rationalize cube roots in other types of limits?
  2. What is the difference between using direct substitution versus rationalization in limits?
  3. How would this process change for a square root instead of a cube root?
  4. Can L'Hopital's Rule be used to solve this limit problem?
  5. What happens when you apply limits at infinity for similar rational expressions?

Tip: When dealing with indeterminate forms, rationalizing can be an effective tool for simplifying expressions, especially with roots.

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Math Problem Analysis

Mathematical Concepts

Limits
Indeterminate Forms
Rationalization
Cube Roots

Formulas

lim (x→a) f(x) / g(x) = 0/0 (Indeterminate Form)
(a - b)(a^2 + ab + b^2) = a^3 - b^3 (Rationalizing the numerator)

Theorems

Limit Theorem
Indeterminate Forms
Rationalization Techniques

Suitable Grade Level

Grades 11-12 (Advanced High School Calculus)