We are asked to find the limit of the function $\frac{(2x-1)^3(x+2)^5}{x^8-1}$ as $x$ approaches infinity. That is, we want to find $$ \lim_{x \to \infty} \frac{(2x-1)^3(x+2)^5}{x^8-1} $$

AnalysisLimitsFunctionsAsymptotic Analysis
2025/5/29

1. Problem Description

We are asked to find the limit of the function (2x1)3(x+2)5x81\frac{(2x-1)^3(x+2)^5}{x^8-1} as xx approaches infinity. That is, we want to find
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, we can divide both the numerator and denominator by the highest power of xx present, which is x8x^8. First, let's consider the highest power of xx in the numerator.
(2x1)3(2x-1)^3 has the highest power x3x^3, with coefficient 23=82^3=8.
(x+2)5(x+2)^5 has the highest power x5x^5, with coefficient 11.
Thus, the numerator has the highest power x3x5=x8x^3 \cdot x^5 = x^8 with coefficient 81=88 \cdot 1 = 8.
We have
limx(2x1)3(x+2)5x81=limx(2x)3(x)5+lower order termsx81=limx8x8+lower order termsx81 \lim_{x \to \infty} \frac{(2x-1)^3(x+2)^5}{x^8-1} = \lim_{x \to \infty} \frac{(2x)^3(x)^5 + \text{lower order terms}}{x^8-1} = \lim_{x \to \infty} \frac{8x^8 + \text{lower order terms}}{x^8-1}
Now, we divide both the numerator and denominator by x8x^8:
limx8+lower order termsx811x8 \lim_{x \to \infty} \frac{8 + \frac{\text{lower order terms}}{x^8}}{1 - \frac{1}{x^8}}
As xx \to \infty, 1x80\frac{1}{x^8} \to 0 and lower order termsx80\frac{\text{lower order terms}}{x^8} \to 0. Therefore,
limx8+lower order termsx811x8=8+010=81=8 \lim_{x \to \infty} \frac{8 + \frac{\text{lower order terms}}{x^8}}{1 - \frac{1}{x^8}} = \frac{8+0}{1-0} = \frac{8}{1} = 8

3. Final Answer

The final answer is
8.

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