Simple concept, complicated equation

Algebra Level 4

k = 1 63 ( x k ) k + 1 ( x + k ) k 1 = 0 \large \prod_{k=1}^{63}\frac{(x-k)^{k+1}}{(x+k)^{k-1}}=0

Find the sum of all real roots of the equation above.


The answer is 2016.

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1 solution

Alex G
Aug 14, 2016

As this is a product, it will equal zero when at least of of the factors is zero. Therefore, we set the term inside the product equal to zero.

( x k ) k + 1 ( x + k ) k 1 = 0 \frac{(x-k)^{k+1}}{(x+k)^{k-1}}=0 ( x k ) k + 1 = 0 (x-k)^{k+1}=0 x k = 0 x-k=0 x = k x=k

Where k k varies among the integers from 1 to 63. To find the sum of the roots, we sum all k's.

k = 1 63 k \sum_{k=1}^{63} k

Using the formula for the sum of the first n integers, we have

k = 1 63 k = 63 64 2 \sum_{k=1}^{63} k = \dfrac{63\cdot 64}{2}

Evaluating, we get 2016 \boxed {2016}

Interesting ! It is just because of my preference and it becomes my signature now.

Tommy Li - 4 years, 10 months ago

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