Infinity /47

Calculus Level 5

lim n , n multiple of 47 sin ( π 4 n ) sin ( 2 π 4 n ) sin ( ( n 47 1 ) π 4 n ) n = ? \lim_{n \rightarrow \infty, n \text{ multiple of } 47} \sqrt [n] {\sin{\left(\dfrac{\pi}{4n}\right)}\sin{\left(\dfrac{2\pi}{4n}\right)}\ldots\sin\left({\dfrac{(\frac n{47} -1)\pi}{4n}}\right)} = \ ?

Note

  1. A total of n 47 1 \frac n{47}-1 terms are being multiplied.
  2. n n is a multiple of 47.


The answer is 0.89735.

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1 solution

Tanishq Varshney
Jul 25, 2015

let

L = lim n sin ( π 4 n ) sin ( 2 π 4 n ) . . . . . . . . . sin ( ( n 47 1 ) π 4 n ) n \large{L=\displaystyle \lim_{n\to \infty} \sqrt [ n ]{ \sin { \left( \frac { \pi }{ 4n } \right) } \sin { \left( \frac { 2\pi }{ 4n } \right) ......... } \sin { \left( \frac { \left( \frac { n }{ 47 } -1 \right) \pi }{ 4n } \right) } } }

ln ( L ) = lim n 1 n r = 1 n 47 1 ln ( sin ( r π 4 n ) ) \large{\ln (L)=\displaystyle \lim_{n\to \infty} \frac{1}{n} \displaystyle \sum^{\frac{n}{47}-1}_{r=1} \ln(\sin (\frac{r \pi}{4n}))}

Reimann integration method

ln L = 0 1 47 ln ( sin ( π 4 x ) ) d x \large{\ln L=\displaystyle \int^{\frac{1}{47}}_{0} \ln(\sin(\frac{\pi}{4}x))dx}

Frankly i used wolfram alpha after this step

ln L = 0.108 \large{\ln L=-0.108}

L = e 0.108 = 0.897 \large{L=e^{-0.108}=\boxed{0.897}}

@Satyajit Mohanty @Pi Han Goh could u guyz help me out with the integral

Tanishq Varshney - 5 years, 10 months ago

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The note to the integral .

Satyajit Mohanty - 5 years, 10 months ago

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