Limits
Limit involving $\sin x\cdot\sin2x\cdots\sin nx$ vs product form
MMTS_Full_Test_03
Grade 12

Question:

If $\displaystyle\lim_{x\to0}\frac{\sin x\sin2x\sin3x\cdots\sin nx-x^n}{x\tan((1+x)(1+2x)(1+3x)\cdots(1+2023x))}$ exists and is non-zero, then $n$ equals
(A) 2022
(B) 2023
(C) 2024
(D) no such $n$

Step-by-Step Solution

Key Concept: Numerator: $\sin x\cdots\sin nx\sim x^n\cdot n!-x^n=x^n(n!-1)$ near 0 — non-zero if $n!\neq1$. Denominator: $x\tan(\prod(1+kx))\sim x\cdot(\prod(1+kx)-1)\sim x\cdot(\sum_{k=1}^{2023}kx)=x\cdot O(x)=O(x^2)$. For limit to exist (and be non-zero), degrees must match.
Step 1: Analyze the denominator of the limit expression as $x \to 0$. The denominator is $D(x) = x\tan((1+x)(1+2x)(1+3x)\cdots(1+2023x))$. Let $P(x) = (1+x)(1+2x)(1+3x)\cdots(1+2023x)$. As $x \to 0$, each term $(1+kx)$ approaches $1$. Thus, $P(x) \to (1)(1)(1)\cdots(1) = 1$ as $x \to 0$. Therefore, $\tan(P(x)) \to \tan(1)$ as $x \to 0$. Since $\tan(1)$ is a non-zero constant, the denominator behaves as $x \tan(1)$ for small $x$. Step 2: Analyze the numerator of the limit expression for positive integer values of $n$. The numerator is $N(x) = \sin x\sin2x\sin3x\cdots\sin nx-x^n$. We use the Taylor series expansion for $\sin(kx)$ around $x=0$: $$ \sin(kx) = kx - \frac{(kx)^3}{3!} + O(x^5) = kx \left(1 - \frac{(kx)^2}{6} + O(x^4)\right) $$ Now, consider the product of these terms: $$ \prod_{k=1}^n \sin(kx) = \prod_{k=1}^n \left(kx \left(1 - \frac{k^2x^2}{6} + O(x^4)\right)\right) $$ $$ = \left(\prod_{k=1}^n kx\right) \left(\prod_{k=1}^n \left(1 - \frac{k^2x^2}{6} + O(x^4)\right)\right) $$ $$ = (n!)x^n \left(1 - \left(\sum_{k=1}^n \frac{k^2x^2}{6}\right) + O(x^4)\right) $$ $$ = n!x^n - \frac{n!}{6} \left(\sum_{k=1}^n k^2\right)x^{n+2} + O(x^{n+4}) $$ Using the sum of squares formula $\sum_{k=1}^n k^2 = \frac{n(n+1)(2n+1)}{6}$, we get: $$ \prod_{k=1}^n \sin(kx) = n!x^n - \frac{n!}{6} \left(\frac{n(n+1)(2n+1)}{6}\right)x^{n+2} + O(x^{n+4}) $$ So the numerator becomes: $$ N(x) = \left(n!x^n - \frac{n!n(n+1)(2n+1)}{36}x^{n+2} + O(x^{n+4})\right) - x^n $$ $$ N(x) = (n! - 1)x^n - \frac{n!n(n+1)(2n+1)}{36}x^{n+2} + O(x^{n+4}) $$ Step 3: Evaluate the limit for $n=1$. For $n=1$, the numerator is $N(x) = (1!-1)x^1 - \frac{1! \cdot 1(1+1)(2\cdot 1+1)}{36}x^{1+2} + O(x^{1+4})$. $$ N(x) = (1-1)x - \frac{1 \cdot 1 \cdot 2 \cdot 3}{36}x^3 + O(x^5) $$ $$ N(x) = 0 \cdot x - \frac{6}{36}x^3 + O(x^5) = -\frac{1}{6}x^3 + O(x^5) $$ The limit is: $$ \lim_{x\to0}\frac{N(x)}{D(x)} = \lim_{x\to0}\frac{-\frac{1}{6}x^3 + O(x^5)}{x\tan(1)} = \lim_{x\to0}\frac{-\frac{1}{6}x^2 + O(x^4)}{\tan(1)} = 0 $$ Thus, for $n=1$, the limit exists but is equal to $0$, which contradicts the condition that the limit is non-zero. Step 4: Evaluate the limit for $n \geq 2$. For $n \geq 2$, the term $n!-1$ is non-zero (e.g., $2!-1=1$, $3!-1=5$). The numerator is $N(x) = (n! - 1)x^n - \frac{n!n(n+1)(2n+1)}{36}x^{n+2} + O(x^{n+4})$. The limit becomes: $$ \lim_{x\to0}\frac{N(x)}{D(x)} = \lim_{x\to0}\frac{(n! - 1)x^n - \frac{n!n(n+1)(2n+1)}{36}x^{n+2} + O(x^{n+4})}{x\tan(1)} $$ $$ = \frac{1}{\tan(1)}\lim_{x\to0}\left((n! - 1)x^{n-1} - \frac{n!n(n+1)(2n+1)}{36}x^{n+1} + O(x^{n+3})\right) $$ Since $n \geq 2$, $n-1 \geq 1$. Therefore, $x^{n-1} \to 0$ as $x \to 0$. All higher order terms ($x^{n+1}$, $x^{n+3}$, etc.) also approach $0$. So, for $n \geq 2$, the limit is $\frac{1}{\tan(1)} \cdot 0 = 0$. Again, for $n \geq 2$, the limit exists but is equal to $0$, contradicting the condition that the limit is non-zero. Step 5: Consider the case for $n < 0$. The phrase "$\sin x\sin2x\sin3x\cdots\sin nx$" typically implies $n$ is a positive integer, as it refers to a product from $k=1$ to $n$. However, if we were to consider $n$ as a negative integer, say $n = -m$ where $m > 0$: The term $x^n$ would become $x^{-m} = \frac{1}{x^m}$. The numerator would involve a term like $\dots - \frac{1}{x^m}$. As $x \to 0$, this term tends to infinity, making the limit non-existent. Step 6: Conclude based on the conditions. For all positive integer values of $n$ ($n \geq 1$), the limit exists and is equal to $0$. If $n$ were allowed to be negative, the limit would be non-existent. The problem requires the limit to exist and be non-zero. Since none of the possible values of $n$ (positive integers, or generally real numbers) satisfy this condition, no such $n$ exists. The final answer is $\boxed{\text{no such value of } n \text{ exists}}$ (Option D).
Correct Answer: (D) no such value of $n$ exists

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