An alternating harmonic series with trigonometry

Calculus Level 5

1000 0 1 sin ( 1 x ) d x = ? \large \left \lfloor 1000 \int_0^1 \left \lceil \sin \left( \frac1x\right) \right \rceil \, dx \right \rfloor = \ ?

You may use a calculator in the final step of your working.


The answer is 779.

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1 solution

Alisa Meier
Jul 29, 2015

The ceiling function is 1 1 , if and only if s i n ( 1 / x ) > 0 sin(1/x) > 0

Therefore the integral becomes a sum of the intervals, where sin 1 x \sin {1 \over x} is non negative, thus one gets:

( 1 1 π ) + ( 1 2 π 1 3 π ) + ( 1 4 π 1 5 π ) + = 1 1 π ( 1 1 2 + 1 3 1 4 + ) = 1 1 π ln ( 1 + 1 ) = 1 ln ( 2 ) π 0.7993... \left(1-\frac{1}{\pi}\right)+\left(\frac{1}{2\pi}-\frac{1}{3\pi}\right)+\left(\frac{1}{4\pi}-\frac{1}{5\pi}\right)+\ldots \\ = 1-\frac{1}{\pi}\left(1-\frac12+\frac13-\frac14+\ldots\right) \\ = 1-\frac{1}{\pi}\ln(1+1)=1-\frac{\ln(2)}{\pi} \approx 0.7993...

Which leads directly to the desired answer: 799 799

One doubt why did u take limits from 1 to 1 π \frac{1}{\pi} not from 1 to 1 2 \frac{1}{2}

Deep seth - 5 years, 9 months ago

How did you evaluate the series 1 1 2 + 1 3 1 4 + 1-\frac12+\frac13-\frac14+\dots ?

Daniel Leite - 5 years, 9 months ago

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Consider the Maclaurin series for ln(1 - x), set x =-1.

Pi Han Goh - 5 years, 7 months ago

@Alisa Meier Typo in the end.......the answer should be 779

Aaghaz Mahajan - 2 years, 12 months ago

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