Equation Anyone?

Algebra Level 3

If α \alpha and β \beta are the roots of the equation λ ( x 2 x ) + x + 5 = 0 \lambda (x^2-x)+x+5=0 and if λ 1 \lambda_1 and λ 2 \lambda_2 are the two values of λ \lambda for which the roots α \alpha and β \beta are connected by the relation α β + β α = 4 5 \dfrac \alpha \beta + \dfrac \beta \alpha = \dfrac 45 , what is λ 1 λ 2 + λ 2 λ 1 = ? \dfrac {\lambda_1}{\lambda_2} +\dfrac{\lambda_2}{\lambda_1}=?


The answer is 254.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

λ ( x 2 x ) + x + 5 = 0 λ x 2 ( λ 1 ) x + 5 = 0 x 2 λ 1 λ x + 5 λ = 0 \begin{aligned} \lambda (x^2-x) + x + 5 & = 0 \\ \lambda x^2- (\lambda - 1)x + 5 & = 0 \\ x^2 - \frac {\lambda -1}\lambda x + \frac 5\lambda & = 0 \end{aligned}

Since α \alpha and β \beta are the roots of the equation, by Vieta's formula , we have: α + β = λ 1 λ \alpha + \beta = \dfrac {\lambda-1}\lambda and α β = 5 λ \alpha \beta = \dfrac 5 \lambda . Now,

α + β = λ 1 λ Squaring both sides ( α + β ) 2 = ( λ 1 λ ) 2 α 2 + 2 α β + β 2 = λ 2 2 λ + 1 λ 2 Dividing both sides by α β = 5 λ α 2 + 2 α β + β 2 α β = λ 2 2 λ + 1 λ 2 5 λ α β + 2 + β α = λ 2 2 λ + 1 5 λ Given that α β + β α = 4 5 2 + 4 5 = λ 2 2 λ + 1 5 λ Rearranging λ 2 16 λ + 1 = 0 \begin{aligned} \alpha + \beta & = \frac {\lambda-1}\lambda & \small \color{#3D99F6} \text{Squaring both sides} \\ (\alpha + \beta)^2 & = \left(\frac {\lambda-1}\lambda\right)^2 \\ \alpha^2 + 2\alpha \beta + \beta^2 & = \frac {\lambda^2-2\lambda +1}{\lambda^2} & \small \color{#3D99F6} \text{Dividing both sides by } \alpha \beta = \frac 5\lambda \\ \frac {\alpha^2 + 2\alpha \beta + \beta^2}{\color{#3D99F6}\alpha \beta} & = \frac {\lambda^2-2\lambda +1}{\lambda^2\cdot \color{#3D99F6} \frac 5 \lambda} \\ {\color{#3D99F6}\frac \alpha \beta} + 2 + \color{#3D99F6}\frac \beta \alpha & = \frac {\lambda^2-2\lambda +1}{5 \lambda} & \small \color{#3D99F6} \text{Given that }\frac \alpha \beta + \frac \beta \alpha = \frac 45 \\ 2 + \color{#3D99F6} \frac 45 & = \frac {\lambda^2-2\lambda +1}{5 \lambda} & \small \color{#3D99F6} \text{Rearranging} \\ \implies \lambda^2-16\lambda +1 & = 0 \end{aligned}

By Vieta's formula, we have λ 1 + λ 2 = 16 \lambda_1 + \lambda_2 = 16 and λ 1 λ 2 = 1 \lambda_1\lambda_2 = 1 and λ 1 λ 2 + λ 2 λ 1 = ( λ 1 + λ 2 ) 2 λ 1 λ 2 2 = 1 6 2 1 2 = 254 \dfrac {\lambda_1}{\lambda_2} + \dfrac {\lambda_2}{\lambda_1} = \dfrac {(\lambda_1+ \lambda_2)^2}{\lambda_1 \lambda_2} - 2 = \dfrac {16^2}1 - 2 = \boxed{254} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...