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Question: Answered & Verified by Expert
If $\alpha, \beta, \gamma$ are the roots of $x^3+p x^2+q x+r=0$, then $\alpha^3+\beta^3+\gamma^3=$
MathematicsQuadratic EquationTS EAMCETTS EAMCET 2018 (04 May Shift 2)
Options:
  • A $p^3-3 p q+r$
  • B $p^2-2 p q+r$
  • C $3 p q-3 r-p^3$
  • D $3 p q+3 r+p^3$
Solution:
1352 Upvotes Verified Answer
The correct answer is: $3 p q-3 r-p^3$
$\alpha, \beta, \gamma$ are roots of $x^3+p x^2+q x+\gamma=0$, then $\alpha^3+\beta^3+\gamma^3=$ ?
$$
\alpha+\beta+\gamma=-p
$$

$$
\begin{array}{lc}
\Rightarrow & \alpha \beta+\beta \gamma+\gamma \alpha=q \\
\Rightarrow & \alpha \beta \gamma=-r \\
\Rightarrow & {[\alpha+\beta+\gamma]^3=\alpha^3+\beta^3+\gamma^3+3(\alpha+\beta+\gamma)} \\
& \quad(\alpha \beta+\beta \gamma+\gamma \alpha)+3 \alpha \beta \gamma \\
\Rightarrow \alpha^3+\beta^3+\gamma^3=(\alpha+\beta+\gamma)^3-3(-p)(q)+3(-r) \\
\Rightarrow & \alpha^3+\beta^3+\gamma^3=-p^3+3 p q-3 r
\end{array}
$$

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