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Geometry Level 4

T n = cot 1 2 + cot 1 8 + cot 1 18 + cot 1 2 n 2 \mathfrak{T}_n=\cot^{-1} 2+\cot^{-1} 8+\cot^{-1} 18+\cdots \cot^{-1} 2n^2

lim n T n = α π γ \displaystyle \lim_{n\to\infty}\mathfrak{T}_n=\dfrac{\color{#D61F06}{\alpha }\pi}{\color{#3D99F6}{\gamma}} where α \color{#D61F06}{\alpha} and γ \color{#3D99F6}{\gamma} are coprime. α + 4 γ = ? \Large \color{#D61F06}{\alpha} +4\color{#3D99F6}{\gamma}=\ ?


The answer is 17.

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2 solutions

Rishabh Jain
Apr 10, 2016

T n = r = 1 n tan 1 ( 1 2 r 2 ) = r = 1 n tan 1 ( 2 4 r 2 ) = r = 1 n tan 1 ( ( 2 r + 1 ) ( 2 r 1 ) 1 + ( 2 r + 1 ) ( 2 r 1 ) ) = r = 1 n tan 1 ( 2 r + 1 ) tan 1 ( 2 r 1 ) \begin{aligned}\mathfrak{T}_n=&\displaystyle\sum_{r=1}^n\tan^{-1}\left( \dfrac{1}{2r^2}\right)\\=&\displaystyle\sum_{r=1}^n\tan^{-1} \left(\dfrac{2}{ 4r^2}\right)\\=&\displaystyle\sum_{r=1}^n\tan^{-1}\left( \dfrac{(2r+1)-(2r-1)}{1+(2r+1)(2r-1)}\right)\\=&\displaystyle\sum_{r=1}^n \tan^{-1}(2r+1)-\tan^{-1}(2r-1)\end{aligned}

( A T e l e s c o p i c S e r i e s ) \large\mathbf{(A~Telescopic~Series)}

T n = tan 1 ( 2 n + 1 ) tan 1 ( 1 ) = tan 1 ( 2 n + 1 ) π 4 \large{ \mathfrak{T}_n=\tan^{-1}(2n+1)-\tan^{-1}(1)~~~~\\=\tan^{-1}(2n+1)-\dfrac{\pi}{4}}~~~

lim n T n = π 2 π 4 = π 4 \large\therefore\displaystyle\lim_{n\to\infty}\mathfrak{T}_n=\dfrac{\pi}{2}-\dfrac{\pi}{4}=\dfrac{\pi}{4}

1 + 4 ( 4 ) = 17 \huge \therefore \color{#D61F06}{1}+4(\color{#3D99F6}{4})=\boxed{17}

Neat solution as usual! =D

Pi Han Goh - 5 years, 2 months ago

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Rishabh Jain - 5 years, 2 months ago

yeah solved it !!! @Rishabh Cool i guess you love summation ???

Rudraksh Sisodia - 5 years, 2 months ago

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Yep.... :-).... Telescopy amazes me....

Rishabh Jain - 5 years, 2 months ago

Same solution @Rishabh Cool .

Vignesh S - 5 years, 2 months ago

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Great.. :-)

Rishabh Jain - 5 years, 2 months ago

S = lim n k = 1 n cot 1 ( 2 k 2 ) S = \displaystyle \lim_{n\rightarrow \infty} \sum_{k=1}^{n} \cot^{-1}(2k^{2})

S = lim n k = 1 n tan 1 ( 1 2 k 2 ) = lim n k = 1 n tan 1 ( ( 2 k + 1 ) ( 2 k 1 ) 1 + ( 2 k + 1 ) ( 2 k 1 ) ) S =\displaystyle \lim_{n\rightarrow \infty} \sum_{k=1}^{n} \tan^{-1}\left(\dfrac{1}{2k^{2}}\right) =\displaystyle \lim_{n\rightarrow \infty} \sum_{k=1}^{n} \tan^{1}\left(\dfrac{(2k+1)-(2k-1)}{1+(2k+1)(2k-1)}\right)
S = lim n k = 1 n tan 1 ( 2 k + 1 ) tan 1 ( 2 k 1 ) \therefore S = \displaystyle \lim_{n \rightarrow \infty} \sum_{k=1}^{n} \tan^{-1}(2k+1) - \tan^{-1}(2k-1)
S = lim n tan 1 ( 3 ) tan 1 ( 1 ) + tan 1 ( 5 ) tan 1 ( 3 ) + tan 1 ( 2 n + 1 ) tan 1 ( 2 n 1 ) = lim n tan 1 ( 2 n + 1 ) tan 1 ( 1 ) = π 2 π 4 = π 4 \therefore S = \displaystyle \lim_{n \rightarrow \infty} \tan^{-1}(3) - \tan^{-1}(1) + \tan^{-1}(5) - \tan^{-1}(3) + \ldots \tan^{-1}(2n+1) - \tan^{-1}(2n-1) = \displaystyle \lim_{n \rightarrow \infty} \tan^{-1}(2n+1) - \tan^{-1}(1) = \dfrac{\pi}{2} -\dfrac{\pi}{4} =\dfrac{\pi}{4}
α + 4 γ = 1 + 4 4 = 17 \alpha + 4\gamma = 1 + 4\cdot 4 = 17


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