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If A=[12221−2m2n]A = \begin{bmatrix} 1 & 2 & 2 \\ 2 & 1 & -2 \\ m & 2 & n \end{bmatrix} is a matrix, such that AAT=9I3AA^T = 9I_3, then the values of mm and nn are (where I3I_3 is an identity matrix of order 3)

Solution

✅ Correct Option: 4

AAT=9IAA^T=9I forces the rows to be mutually orthogonal with squared length 9. Row3 with Row1: m+4+2n=0m+4+2n=0; Row3 with Row2: 2m+2−2n=02m+2-2n=0, i.e. n=m+1n=m+1. Solving, 3m+6=03m+6=0, so m=−2m=-2 and n=−1n=-1, which also satisfies m2+4+n2=9m^2+4+n^2=9.

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