I Was Very Amazed At The Solution 9

Algebra Level 3

16 α 360 n = 1 ( n 1 ) ! i = 1 n ( α + i ) \large 16 \sum_{\alpha \mid 360} \sum_{n = 1}^{\infty} \dfrac{(n-1)!}{\prod_{i = 1}^{n} ( \alpha + i )}

Evaluate the expression above, where α \alpha are positive factors of 360.

For more problems like this, try this set and easier set .


The answer is 52.

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1 solution

Christian Daang
Jul 19, 2017

I used the theorem that for any α \alpha ,

n = 1 ( n 1 ) ! i = 1 n ( α + i ) = 1 α \sum_{n = 1}^{\infty} \dfrac{(n-1)!}{\displaystyle \prod_{i = 1}^{n} ( \alpha + i )} = \dfrac{1}{\alpha} \cdot

Hence, the equation above can be simplified to:

16 ( α positive factors of 360 1 α ) 16\left( \sum_{\alpha \ \in \ \text{{positive factors of 360}}} \dfrac{1}{\alpha} \right ) \cdot

But, as α positive factors of 360 1 α = sum of positive factors of 360 360 = 1170 360 , \displaystyle \sum_{\alpha \ \in \ \text{{positive factors of 360}}} \dfrac{1}{\alpha} = \dfrac{\text{sum of positive factors of 360}}{360} = \dfrac{1170}{360} \ ,

16 ( α positive factors of 360 ( n = 1 ( n 1 ) ! i = 1 n ( α + i ) ) ) = 16 ( 1170 360 ) = 52 \begin{aligned} \implies 16 \left( \sum_{\alpha \ \in \ \text{{positive factors of 360}}} \left(\sum_{n = 1}^{\infty} \dfrac{(n-1)!}{\displaystyle \prod_{i = 1}^{n} ( \alpha + i )} \right)\right) \\ &= 16\left(\dfrac{1170}{360}\right) \\ &= \boxed{52} \end{aligned}

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