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# If $\alpha, \beta, \gamma$ are the roots of $x^3+2x^2-3x-1=0$, then $\alpha^{-2}+\beta^{-2}+\gamma^{-2}=$

$\begin {array} {1 1} (a)\;12 & \quad (b)\;13 \\ (c)\;14 & \quad (d)\;15 \end {array}$

(b) 13
Can you explain

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