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

AnalysisLimitsTrigonometryCalculusL'Hopital's Rule (Implicitly)Series Expansion (Implicitly)
2025/5/27

1. Problem Description

We are asked to evaluate the limit:
limx+x2(1cos(1x))\lim_{x \to +\infty} x^2 (1 - \cos(\frac{1}{x}))

2. Solution Steps

Let u=1xu = \frac{1}{x}. As x+x \to +\infty, we have u0u \to 0. Therefore, the limit can be rewritten as:
limu01u2(1cos(u))\lim_{u \to 0} \frac{1}{u^2} (1 - \cos(u))
We can multiply the expression by 1+cos(u)1+cos(u)\frac{1 + \cos(u)}{1 + \cos(u)} to get:
limu01u2(1cos(u))(1+cos(u))1+cos(u)=limu01u21cos2(u)1+cos(u)\lim_{u \to 0} \frac{1}{u^2} \frac{(1 - \cos(u))(1 + \cos(u))}{1 + \cos(u)} = \lim_{u \to 0} \frac{1}{u^2} \frac{1 - \cos^2(u)}{1 + \cos(u)}
Using the identity sin2(u)+cos2(u)=1\sin^2(u) + \cos^2(u) = 1, we have 1cos2(u)=sin2(u)1 - \cos^2(u) = \sin^2(u).
So the limit becomes:
limu0sin2(u)u211+cos(u)=limu0(sin(u)u)211+cos(u)\lim_{u \to 0} \frac{\sin^2(u)}{u^2} \frac{1}{1 + \cos(u)} = \lim_{u \to 0} \left(\frac{\sin(u)}{u}\right)^2 \frac{1}{1 + \cos(u)}
We know that limu0sin(u)u=1\lim_{u \to 0} \frac{\sin(u)}{u} = 1 and limu0cos(u)=1\lim_{u \to 0} \cos(u) = 1. Thus,
limu0(sin(u)u)211+cos(u)=(1)211+1=112=12\lim_{u \to 0} \left(\frac{\sin(u)}{u}\right)^2 \frac{1}{1 + \cos(u)} = (1)^2 \frac{1}{1 + 1} = 1 \cdot \frac{1}{2} = \frac{1}{2}

3. Final Answer

12\frac{1}{2}

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