Question

Let \( f : (-\infty, \infty) - \{0\} \to \mathbb{R} \) be a differentiable function such that \( f'(1) = \lim_{a \to \infty} a^2 f\left(\frac{1}{a}\right) \). Then \( \lim_{a \to \infty} \frac{a(a + 1)}{2} \tan^{-1}\left(\frac{1}{a}\right) + a^2 - 2 \log_e a \) is equal to:

• A. \( \frac{3}{2} + \frac{\pi}{4} \)
• B. \( \frac{3}{8} + \frac{\pi}{4} \)
• C. \( \frac{5}{2} + \frac{\pi}{8} \)
• D. \( \frac{3}{4} + \frac{\pi}{8} \)

Answer 1

The Correct Option is C

We are given the limit expression:

\[ \lim_{a \to \infty} \frac{a(a + 1)}{2} \tan^{-1} \left( \frac{1}{a} \right) + a^2 - 2 \ln a. \]

Step 1: Simplify the \(\tan^{-1}\) term Using the expansion:

\[ \tan^{-1} \left( \frac{1}{a} \right) \approx \frac{1}{a} - \frac{1}{3a^3} \quad \text{as } a \to \infty. \]

Substitute this approximation:

\[ \frac{a(a + 1)}{2} \tan^{-1} \left( \frac{1}{a} \right) \approx \frac{a(a + 1)}{2} \left( \frac{1}{a} - \frac{1}{3a^3} \right). \]

As \(a \to \infty\), the dominant term is:

\[ \frac{(a + 1)}{2} - \frac{(a + 1)}{6a^2}. \]

As \(a \to \infty\), the dominant term is:

\[ \frac{(a + 1)}{2} \to \frac{a}{2}. \]

Step 2: Rewrite the full limit expression The given expression becomes:

\[ \lim_{a \to \infty} \left( \frac{a}{2} + a^2 - 2 \ln a \right). \]

Step 3: Identify \(f(x)\) From the problem:

\[ f(x) = \frac{1}{2} \left( (1 + x) \tan^{-1}(x) + 1 - 2x^2 \ln(x) \right). \]

Compute \(f'(x)\):

\[ f'(x) = \frac{1}{2} \left( \frac{1 + x}{1 + x^2} + \tan^{-1}(x) + 4x \ln(x) + 2x \right). \]

Substitute \(x = 1\):

\[ f'(1) = \frac{1}{2} \left( \frac{1 + 1}{1 + 1} + \frac{\pi}{4} + 4(1) \ln(1) + 2(1) \right). \]

Simplify:

\[ f'(1) = \frac{1}{2} \left( 1 + \frac{\pi}{4} + 2 \right). \]

Thus:

\[ f'(1) = \frac{5}{2} + \frac{\pi}{8}. \]