Vieta's might be practical

Algebra Level 5

1 x 1 + 2 x 2 + 3 x 3 + 4 x 4 + 5 x 5 = 0 1 x^1 + 2x^2 + 3x^3 + 4x^4 + 5x^5 = 0

Let a 1 , a 2 , , a 5 a_1, a_2, \ldots,a_{5} denote the roots to the above equation. Find the value:

( 4 a 1 + 1 ) ( 4 a 2 + 1 ) ( 4 a 5 + 1 ) (4{ a }_{ 1 }+1)(4a_{ 2 }+1)\cdots (4a_{ 5 }+1)


Inspiration .


The answer is 33.

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2 solutions

Chew-Seong Cheong
Mar 14, 2016

Since a 1 a_1 , a 2 a_2 , ..., a 5 a_5 are the roots of the summation, which means: m = 1 5 m x m = 5 m = 1 5 ( x a m ) \begin{aligned} \sum_{m=1}^5 mx^m & = 5 \prod_{m=1}^5 (x-a_m) \end{aligned}

Now, we have:

m = 1 5 ( 4 a m + 1 ) = 4 5 m = 1 5 ( a m + 1 4 ) = 4 5 m = 1 5 ( 1 4 a m ) = 4 5 5 m = 1 5 m ( 1 4 ) m = 1 5 m = 1 5 ( 4 ) 5 m m = 165 5 = 33 \begin{aligned} \prod_{m=1}^5 (4a_m + 1) & = 4^5 \prod_{m=1}^5 \left(a_m + \frac{1}{4} \right) \\ & = - 4^5 \prod_{m=1}^5 \left(-\frac{1}{4} - a_m \right) \\ & = - \frac{4^5}{5} \sum_{m=1}^5 m\left(-\frac{1}{4} \right)^m \\ & = \frac{1}{5} \sum_{m=1}^5 (-4)^{5-m} m \\ & = \frac{165}{5} = \boxed{33} \end{aligned}

Sarith Imaduwage
Apr 7, 2016

Correct! +1!

Joel Yip - 5 years, 2 months ago

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