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If the matrix [2−13λ07−114]\begin{bmatrix}2 & -1 & 3\\ \lambda & 0 & 7\\-1 & 1 & 4\end{bmatrix} is not invertible, then value of λ\lambda is

Solution

✅ Correct Option: 3

A matrix is not invertible when its determinant equals 0.

For the matrix [2−13λ07−114]\begin{bmatrix}2 & -1 & 3\\ \lambda & 0 & 7\\-1 & 1 & 4\end{bmatrix}, expanding along the first row:

det(A)=2∣0714∣−(−1)∣λ7−14∣+3∣λ0−11∣\text{det}(A) = 2 \begin{vmatrix}0 & 7\\1 & 4\end{vmatrix} - (-1) \begin{vmatrix}\lambda & 7\\-1 & 4\end{vmatrix} + 3 \begin{vmatrix}\lambda & 0\\-1 & 1\end{vmatrix}


For the 2×2 determinants:

∣0714∣=(0)(4)−(7)(1)\begin{vmatrix}0 & 7\\1 & 4\end{vmatrix} = (0)(4) - (7)(1)

=−7= -7

∣λ7−14∣=(λ)(4)−(7)(−1)\begin{vmatrix}\lambda & 7\\-1 & 4\end{vmatrix} = (\lambda)(4) - (7)(-1)

=4λ+7= 4\lambda + 7

∣λ0−11∣=(λ)(1)−(0)(−1)\begin{vmatrix}\lambda & 0\\-1 & 1\end{vmatrix} = (\lambda)(1) - (0)(-1)

=λ= \lambda


Substituting back:

det(A)=2(−7)+1(4λ+7)+3(λ)\text{det}(A) = 2(-7) + 1(4\lambda + 7) + 3(\lambda)

=−14+4λ+7+3λ= -14 + 4\lambda + 7 + 3\lambda

=−7+7λ= -7 + 7\lambda


Since the matrix is not invertible:

−7+7λ=0-7 + 7\lambda = 0

7λ=77\lambda = 7

λ=1\lambda = 1

Therefore, the value of λ\lambda is 11.

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