Will you divide the limit till this year?

Calculus Level 5

If 0 2014 tan 1 x d x = a cot b \displaystyle \int_{0}^{2014}\left \lfloor \tan^{-1}x \right \rfloor dx = a - \cot b , what is a b \left \lfloor \dfrac{a}{b}\right \rfloor ?

Notation: \lfloor \cdot \rfloor denotes the floor function .

Please post your solution.


The answer is 3528.

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2 solutions

U Z
Oct 27, 2014

= 0 t a n 1 0 + t a n 1 2014 1 = \displaystyle \int_{0}^{tan1} 0 + \displaystyle \int_{tan1}^{2014} 1

= 2014 t a n 1 = 2014 - tan1

In the expression tan1, 1 is in radians. So while changing tan in to cot, it should be cot(pi/2 - 1) which will leave us with 'b' as 8/14.

Somdutt Goyal - 6 years, 7 months ago

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Oh sorry for that just wanted to make this problem a Level 2

U Z - 6 years, 7 months ago

Yes with that answer comes 3528

Krishna Sharma - 6 years, 7 months ago

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Can you help me how to change the answer @Krishna Sharma @Somdutt Goyal

U Z - 6 years, 7 months ago

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@U Z I guess only moderator and staff can change

Krishna Sharma - 6 years, 7 months ago

@U Z I have updated the answer to 3528, and reverted the change you made to the problem.

To get the answer updated, you can report your problem as having a wrong answer, and specify what you want to correct it to (and why).

Calvin Lin Staff - 6 years, 7 months ago
Chew-Seong Cheong
Aug 30, 2018

Note that tan 1 x \tan^{-1} x is an increasing function for x [ 0 , 2014 ] x \in [0, 2014] and that tan 1 2014 1.570 \tan^{-1} 2014 \approx 1.570 . Then we have:

I = 0 2014 tan 1 x d x = 0 tan 1 tan 1 x d x + tan 1 2014 tan 1 x d x = 0 + tan 1 2014 d x = 2014 tan 1 = 2014 cot ( π 2 1 ) \begin{aligned} I & = \int_0^{2014} \left \lfloor \tan^{-1} x \right \rfloor dx \\ & = \int_0^{\tan 1} \left \lfloor \tan^{-1} x \right \rfloor dx + \int_{\tan 1}^{2014} \left \lfloor \tan^{-1} x \right \rfloor dx \\ & = 0 + \int_{\tan 1}^{2014} dx \\ & = 2014 - \tan 1 \\ & = 2014 - \cot \left(\frac \pi 2 - 1\right) \end{aligned}

Therefore, a b = 2014 π 2 1 = 3528 \left \lfloor \dfrac ab \right \rfloor = \left \lfloor \dfrac {2014}{\frac \pi 2 - 1} \right \rfloor = \boxed{3528} .

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