An algebra problem by Priyanshu Mishra

Algebra Level 4

n = 2 n 3 1 n 3 + 1 \large\ \prod _{ n=2 }^{ \infty }{ \frac { { n }^{ 3 } - 1 }{ { n }^{ 3 } + 1 } } .

Find the value of the closed form of the above product.

Give your answer to 3 decimal places.


The answer is 0.667.

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1 solution

Jason Chrysoprase
Jan 13, 2017

n = 2 n 3 1 n 3 + 1 = n = 2 n 1 n + 1 × n = 2 n 2 + n + 1 n 2 n 1 n 3 1 = ( n 1 ) ( n 2 + n + 1 ) , n 3 + 1 = ( n + 1 ) ( n 2 n + 1 ) = ( 1 3 × 2 4 × 3 5 × 4 6 × 5 7 × . . . ) × ( 7 3 × 13 7 × 21 13 × 31 21 × 43 31 × . . . ) By using Telescoping Series = 2 × 1 3 = 2 3 \begin{aligned} \Large \prod_{n=2}^{\infty} \frac{\color{#D61F06}{n^3 -1}}{\color{#3D99F6}{n^3+1}} & \Large = \prod_{n=2}^{\infty} \frac{\color{#D61F06}{n-1}}{\color{#3D99F6}{n+1}} \times \prod_{n=2}^{\infty} \frac{\color{#D61F06}{n^2 + n+1}}{\color{#3D99F6}{n^2 -n-1}} & \large \color{#D61F06}{n^3 - 1 = (n-1)(n^2 +n+1)} \color{#333333}{,} \ \large \color{#3D99F6}{ n^3 + 1 = (n+1)(n^2-n+1)} \\ & \Large = {\left(\frac{1}{{\color{#D61F06} \cancel{3}}} \times \frac{2}{{\color{#3D99F6}\cancel{4}}} \times \frac{\color{#D61F06}{\cancel{3}}}{\color{#D61F06}{\cancel{5}}} \times \frac{\color{#3D99F6}{\cancel{4}}}{\color{#3D99F6}{\cancel{6}}} \times \frac{\color{#D61F06}{\cancel{5}}}{\color{#D61F06}{\cancel{7}}} \times ... \right)} \times {\left(\frac{\color{#D61F06}{\cancel{7}}}{3} \times \frac{\color{#3D99F6}{\cancel{13}}}{{\color{#D61F06}\cancel{7}}} \times \frac{\color{#D61F06}{\cancel{21}}}{\color{#3D99F6}{\cancel{13}}} \times \frac{\color{#3D99F6}{\cancel{31}}}{\color{#D61F06}{\cancel{21}}} \times \frac{\color{#D61F06}{\cancel{43}}}{\color{#D61F06}{\cancel{31}}} \times ... \right)} & \large \color{#3D99F6}{\text{By using Telescoping Series}}\\ & \Large = \large 2 \times \frac{1}{3} \\ & \Large = \large \color{#69047E}{\boxed{\frac{2}{3}}} \\ \end{aligned}

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