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The integer nn for which lim⁡x→0(sin⁡x−x)tan⁡xxn\lim_{x \to 0} \frac{(\sin x - x)\tan x}{x^n} is a finite non zero number, is

Solution

✅ Correct Option: 3

Looking at this limit problem, we need to determine the value of nn that makes the limit finite and non-zero.

To analyze the behavior near x=0x = 0, we'll use Taylor series expansions:

sin⁡x=x−x33!+x55!−x77!+...\sin x = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \frac{x^7}{7!} + ...

sin⁡x=x−x36+x5120−...\sin x = x - \frac{x^3}{6} + \frac{x^5}{120} - ...

Therefore:

sin⁡x−x=−x36+x5120−...\sin x - x = -\frac{x^3}{6} + \frac{x^5}{120} - ...


For tan⁡x\tan x:

tan⁡x=x+x33+2x515+...\tan x = x + \frac{x^3}{3} + \frac{2x^5}{15} + ...


Now we compute the product:

(sin⁡x−x)tan⁡x=(−x36+x5120−...)(x+x33+...)(\sin x - x)\tan x = \left(-\frac{x^3}{6} + \frac{x^5}{120} - ...\right)\left(x + \frac{x^3}{3} + ...\right)

Expanding the leading terms:

(sin⁡x−x)tan⁡x=−x36⋅x−x36⋅x33+x5120⋅x+...(\sin x - x)\tan x = -\frac{x^3}{6} \cdot x - \frac{x^3}{6} \cdot \frac{x^3}{3} + \frac{x^5}{120} \cdot x + ...

(sin⁡x−x)tan⁡x=−x46−x618+x6120+...(\sin x - x)\tan x = -\frac{x^4}{6} - \frac{x^6}{18} + \frac{x^6}{120} + ...

(sin⁡x−x)tan⁡x=−x46+higher order terms(\sin x - x)\tan x = -\frac{x^4}{6} + \text{higher order terms}


The numerator behaves like −x46-\frac{x^4}{6} as x→0x \to 0.

For the limit to be finite and non-zero, we need the denominator to have the same order as the numerator.

Therefore n=4n = 4.


We can verify:

lim⁡x→0(sin⁡x−x)tan⁡xx4=lim⁡x→0−x46+higher order termsx4=−16\lim_{x \to 0} \frac{(\sin x - x)\tan x}{x^4} = \lim_{x \to 0} \frac{-\frac{x^4}{6} + \text{higher order terms}}{x^4} = -\frac{1}{6}

This is indeed finite and non-zero.

Therefore, the answer is n=4n = 4.

Note: The stated correct answer is Option 1 (3), which appears to be inconsistent with the mathematical derivation shown above. The calculation demonstrates that n=4n = 4 (Option 3) yields a finite non-zero limit.

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