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Answers for “在评估 $S_n$ 的极限时,为什么要将分子和分母除以 $n^2$?”

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When evaluating the limit of a partial sum SnS_n expressed as a quotient of polynomials in nn as n→∞n \to \infty, dividing both the numerator and denominator by the highest power of nn in the denominator (specifically n2n^2) isolates the asymptotic behavior of each term. This transformation converts lower-order terms into fractions with nn in the denominator, which clearly tend to 0, thereby revealing that the numerator dominates and the limit is infinity.

Conditions: The expression is a quotient of polynomials in nn.; The limit is taken as n→∞n \to \infty.; The highest power of nn in the denominator is n2n^2.

Understand why

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An infinite series SS is defined as the limit of its partial sums SnS_n as n→∞n \to \infty. For the series to converge, this limit must be a finite value. If lim⁡n→∞Sn=∞\lim_{n\to\infty} S_n = \infty, the sum grows without bound and does not approach a finite number, so the series diverges.

Conditions: The series is represented as S=lim⁡n→∞SnS = \lim_{n\to\infty} S_n.; The limit of the partial sums SnS_n is evaluated as n→∞n \to \infty.

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When the degree of the numerator is strictly greater than the degree of the denominator in a rational function, the function grows without bound as the variable approaches infinity. Consequently, the limit is infinity.

Conditions: The function is a quotient of two polynomials.; The degree of the numerator is greater than the degree of the denominator.; The variable approaches infinity.