Multiple choice

If $\displaystyle \Sigma\alpha=\frac{2}{3},\Sigma\alpha\beta=\frac{4}{3}$ and $\displaystyle \alpha\beta\gamma=\frac{8}{3}$, then the equation whose roots are $\alpha,\beta,\gamma$, is

  1. $x^{3}-2x^{2}+4x-8=0$
  2. $3x^{3}+2x^{2}-4x+8=0$
  3. $3x^{3}-2x^{2}+4x-8=0$
  4. $x^{3}+2x^{2}-4x+8=0$
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C Correct answer
Explanation

The cubic equation with roots alpha, beta, gamma is x^3 - (sum)x^2 + (sum of products)x - (product) = 0. x^3 - (2/3)x^2 + (4/3)x - (8/3) = 0. Multiply by 3: 3x^3 - 2x^2 + 4x - 8 = 0.

AI explanation

By Vieta's formulas, a monic cubic polynomial with roots alpha, beta, and gamma is given by x cubed minus (sum of the roots)x squared plus (sum of the product of roots two at a time)x minus (product of the roots) equals 0. Substituting the given values, the equation becomes x cubed minus (two thirds)x squared plus (four thirds)x minus (eight thirds) equals 0. Multiplying the entire equation by 3 to clear the denominators yields 3x cubed minus 2x squared plus 4x minus 8 equals 0.