Quadratic Equations#0

Algebra Level 3

If the larger root of ( 2015 x ) 2 ( 2014 ) ( 2016 ) x 1 = 0 (2015x)^2 - (2014)(2016)x - 1 = 0 is α \alpha and the smaller root of x 2 + 2014 x 2015 = 0 x^2 + 2014x - 2015 = 0 is β \beta , then find value of α β 224 \sqrt{\dfrac{\alpha-\beta}{224}} .


The answer is 3.

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1 solution

Chew-Seong Cheong
Jan 14, 2018

Consider equation 1:

( 2015 x ) 2 ( 2014 ) ( 2016 ) x 1 = 0 Let n = 2015 n 2 x 2 ( n 1 ) ( n + 1 ) x 1 = 0 n 2 x 2 ( n 2 1 ) x 1 = 0 ( n 2 x + 1 ) ( x 1 ) = 0 \begin{aligned} (2015x)^2 - (2014)(2016)x - 1 & = 0 & \small \color{#3D99F6} \text{Let }n = 2015 \\ n^2x^2 - (n-1)(n+1)x - 1 & = 0 \\ n^2x^2 - (n^2-1)x - 1 & = 0 \\ (n^2x+1)(x-1) & = 0 \end{aligned}

x = { 1 n 2 1 α = 1 \implies x = \begin{cases} - \dfrac 1{n^2} \\ 1 \end{cases} \implies \alpha = 1

Consider equation 2:

x 2 + 2014 x 2015 = 0 x 2 + ( n 1 ) x a = 0 ( x + n ) ( x 1 ) = 0 \begin{aligned} x^2 +2014x - 2015 & = 0 \\ x^2 + (n-1)x-a & = 0 \\ (x+n)(x-1) & = 0 \end{aligned}

x = { n 1 β = n = 2015 \implies x = \begin{cases} -n \\ 1 \end{cases} \implies \beta = -n = - 2015

Therefore, α β 224 = 1 2015 224 = 9 = 3 \sqrt {\dfrac {\alpha - \beta}{224}} = \sqrt {\dfrac {1 - 2015}{224}} = \sqrt 9 = \boxed{3} .

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