Câu 4. Cho \( S_n = \frac{1}{n} + \frac{2}{n} + \ldots + \frac{n}{n} \). a) \( S_3 = 1 \). b) \( \lim_{n \to \infty} \left( 2 S_n - \sqrt{n^2 + n + 1} \right) = \frac{1}{2} \). c) \( 2025 \cdot S_{2025} = 2051325 \). d) \( S_n = \frac{n(n+1)}{2} \)

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Câu 4. Cho \( S_n = \frac{1}{n} + \frac{2}{n} + \ldots + \frac{n}{n} \).

a) \( S_3 = 1 \).

b) \( \lim_{n \to \infty} \left( 2 S_n - \sqrt{n^2 + n + 1} \right) = \frac{1}{2} \).

c) \( 2025 \cdot S_{2025} = 2051325 \).

d) \( S_n = \frac{n(n+1)}{2} \).