We are asked to evaluate the limit: $\lim_{x \to +\infty} \frac{(2x-1)^3(x+2)^5}{x^8-1}$

AnalysisLimitsCalculusAsymptotic Analysis
2025/5/29

1. Problem Description

We are asked to evaluate the limit:
limx+(2x1)3(x+2)5x81\lim_{x \to +\infty} \frac{(2x-1)^3(x+2)^5}{x^8-1}

2. Solution Steps

To evaluate the limit as xx approaches infinity, we can focus on the highest power of xx in both the numerator and the denominator.
First, let's consider the numerator: (2x1)3(x+2)5(2x-1)^3(x+2)^5.
(2x1)3=(2x)3+=8x3+(2x-1)^3 = (2x)^3 + \cdots = 8x^3 + \cdots
(x+2)5=x5+(x+2)^5 = x^5 + \cdots
So, (2x1)3(x+2)5=(8x3+)(x5+)=8x8+(2x-1)^3(x+2)^5 = (8x^3 + \cdots)(x^5 + \cdots) = 8x^8 + \cdots
The leading term in the numerator is 8x88x^8.
Now, let's look at the denominator: x81x^8 - 1.
The leading term in the denominator is x8x^8.
Thus,
limx+(2x1)3(x+2)5x81=limx+8x8+x81\lim_{x \to +\infty} \frac{(2x-1)^3(x+2)^5}{x^8-1} = \lim_{x \to +\infty} \frac{8x^8 + \cdots}{x^8 - 1}
We can divide both the numerator and the denominator by x8x^8:
limx+8+/x811/x8\lim_{x \to +\infty} \frac{8 + \cdots/x^8}{1 - 1/x^8}
As x+x \to +\infty, terms like 1x8\frac{1}{x^8} approach

0. $\lim_{x \to +\infty} \frac{8 + \cdots/x^8}{1 - 1/x^8} = \frac{8 + 0}{1 - 0} = \frac{8}{1} = 8$

3. Final Answer

The limit is
8.

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