Question:medium

Evaluate \[ \lim_{n\to\infty}\sqrt{2} \left[ \frac{(2+\sqrt{2})^n+(2-\sqrt{2})^n} {(2+\sqrt{2})^n-(2-\sqrt{2})^n} \right] \]

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If \[ |r|\lt 1, \] then \[ r^n\to 0 \quad \text{as} \quad n\to\infty. \] In limits involving powers, divide by the dominant term to simplify the expression.
Updated On: Jun 24, 2026
  • \(2+\sqrt{2}\)
  • \(2-\sqrt{2}\)
  • \(1\)
  • \(\sqrt{2}\)
Show Solution

The Correct Option is D

Solution and Explanation

Step 1: Identify the dominant term.
As $n \to \infty$, compare $2 + \sqrt{2} \approx 3.41$ and $2 - \sqrt{2} \approx 0.59$. Since $2 + \sqrt{2} > 1$ and $|2 - \sqrt{2}| < 1$, we have $(2+\sqrt{2})^n \to \infty$ and $(2-\sqrt{2})^n \to 0$.

Step 2: Divide numerator and denominator by $(2+\sqrt{2})^n$.
Let $r = \dfrac{2-\sqrt{2}}{2+\sqrt{2}}$. Then $|r| < 1$, so $r^n \to 0$. The expression becomes: \[ \sqrt{2} \cdot \frac{1 + r^n}{1 - r^n}. \]

Step 3: Take the limit as $n \to \infty$.
Since $r^n \to 0$: \[ \sqrt{2} \cdot \frac{1 + 0}{1 - 0} = \sqrt{2}. \]

Step 4: Verify by computing $r$.
$r = \dfrac{2-\sqrt{2}}{2+\sqrt{2}} = \dfrac{(2-\sqrt{2})^2}{(2+\sqrt{2})(2-\sqrt{2})} = \dfrac{6-4\sqrt{2}}{2} = 3 - 2\sqrt{2} \approx 0.17$. Indeed $|r| < 1$.

Step 5: Confirm $r^n \to 0$.
$r \approx 0.17 < 1$, so $r^n \to 0$ as $n \to \infty$. The computation is valid.

Step 6: State the answer.
The limit is $\sqrt{2}$.
\[ \boxed{\sqrt{2}} \]
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