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4-2.Quadratic Equations and Inequations
hard
If $\alpha, \beta $ and $\gamma$ are the roots of equation ${x^3} - 3{x^2} + x + 5 = 0$ then $y = \sum {\alpha ^2} + \alpha \beta \gamma $ satisfies the equation
A
${y^3} + y + 2 = 0$
B
${y^3} - {y^2} - y - 2 = 0$
C
${y^3} + 3{y^2} - y - 3 = 0$
D
${y^3} + 4{y^2} + 5y + 20 = 0$
Solution
(b) Given equation ${x^3} – 3{x^2} + x + 5 = 0$.
Then $\alpha + \beta + \gamma = 3$, $\alpha \beta + \beta \gamma + \gamma \alpha = 1$, $\alpha \beta \gamma = – 5$
$y = \Sigma {\alpha ^2} + \alpha \beta \gamma = {(\alpha + \beta + \gamma )^2} – 2\,(\alpha \beta + \beta \gamma + \gamma \alpha ) + \alpha \beta \gamma $
= $9 – 2 – 5 = 2$
$\therefore $ $y = 2$
It satisfies the equation ${y^3} – {y^2} – y – 2 = 0$.
Standard 11
Mathematics