Math Problem Statement

Limit of ³√x minus 2 divided by x minus 8 as x approaches 8

Solution

To find the limit of x32x8\frac{\sqrt[3]{x} - 2}{x - 8} as xx approaches 8, we'll follow these steps:

Step 1: Direct Substitution

First, substitute x=8x = 8 directly into the expression:

83288=220=00\frac{\sqrt[3]{8} - 2}{8 - 8} = \frac{2 - 2}{0} = \frac{0}{0}

This results in an indeterminate form 00\frac{0}{0}, so we need to use another method to evaluate the limit.

Step 2: Rationalize the Numerator

To resolve the indeterminate form, we can multiply the numerator and the denominator by the conjugate of the numerator's cube root, using the identity:

a3b3=(ab)(a2+ab+b2)a^3 - b^3 = (a - b)(a^2 + ab + b^2)

Let a=x3a = \sqrt[3]{x} and b=2b = 2. Then:

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

So,

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

Since x8=(x32)(x23+2x3+4)x - 8 = (\sqrt[3]{x} - 2)(\sqrt[3]{x^2} + 2\sqrt[3]{x} + 4), the expression simplifies to:

[ \frac{1}{\sqrt[3]{x^2} + 2\sqrt[3]{x} + 4} ]

Step 3: Take the Limit

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

[ \lim_{x \to 8} \frac{1}{\sqrt[3]{x^2} + 2\sqrt[3]{x} + 4} = \frac{1}{\sqrt[3]{8^2} + 2\sqrt[3]{8} + 4} = \frac{1}{\sqrt[3]{64} + 2 \times 2 + 4} = \frac{1}{4 + 4 + 4} = \frac{1}{12} ]

Final Answer

The limit is:

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

Would you like any details or have further questions?

Here are five related questions you might find interesting:

  1. How would you evaluate limx1x31x1\lim_{x \to 1} \frac{x^3 - 1}{x - 1}?
  2. Can L'Hôpital's Rule be used to solve this limit problem?
  3. What is the geometric interpretation of limits in calculus?
  4. How do you find the limit of a function as xx approaches infinity?
  5. What are other methods to resolve indeterminate forms besides rationalization?

Tip: When you encounter indeterminate forms, always consider algebraic manipulation, such as rationalization or factoring, before resorting to more advanced techniques.

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Math Problem Analysis

Mathematical Concepts

Limits
Rationalization
Indeterminate Forms

Formulas

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Theorems

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Suitable Grade Level

Undergraduate