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If A=[12221−2α2β]A=\begin{bmatrix}1 & 2 & 2\\2 & 1 & -2\\\alpha & 2 & \beta\end{bmatrix} and AAT=9IAA^T=9I, where I is the identity matrix of order 3, then (β−α)(\beta-\alpha) equals:

Solution

✅ Correct Option: 2

AAT=9IAA^T=9I forces the rows to be mutually orthogonal.

Row1 with Row3: α+4+2β=0\alpha+4+2\beta=0.

Row2 with Row3: 2α+2−2β=02\alpha+2-2\beta=0, so β=α+1\beta=\alpha+1.

Substituting, 3α+6=03\alpha+6=0 gives α=−2\alpha=-2, β=−1\beta=-1, and β−α=1\beta-\alpha=1.

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