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If ∣110x2+2x+210211∣=0\left|\begin{array}{ccc}1 & 1 & 0 \\x^2+2 x+2 & 1 & 0 \\2 & 1 & 1\end{array}\right|=0​1x2+2x+22​111​001​
Question

If 110x2+2x+210211=0\left|\begin{array}{ccc}1 & 1 & 0 \\x^2+2 x+2 & 1 & 0 \\2 & 1 & 1\end{array}\right|=0​, then the value of xx​ is

A.

2

B.

-2

C.

1

D.

-1

Correct option is D

Let the matrix be:Δ=110x2+2x+211211We can expand along the third column since it has two zeros, making the calculation easier.Δ=0M131M23+1M33Where Mij is the minor of element (i,j), i.e., determinant of the 2×2 matrix formed by deleting the ith rowand jth column:M33=11x2+2x+21=(1)(1)(1)(x2+2x+2)=1(x2+2x+2)=1x22x2=x22x1Thus:Δ=(11121)+111x2+2x+21)Δ=(1112)+(111(x2+2x+2))Δ=(12)+(1x22x2)Δ=(1)+(x22x1)Δ=1x22x1Δ=x22xSet determinant to zero:x22x=0 x2+2x=0 x(x+2)=0 x=0 or x=2\begin{aligned}&\text{Let the matrix be:} \\&\Delta = \left|\begin{array}{ccc}1 & 1 & 0 \\x^2 + 2x + 2 & 1 & 1 \\2 & 1 & 1\end{array}\right| \\\\&\text{We can expand along the third column since it has two zeros, making the calculation easier.} \\\\&\Delta = 0 \cdot M_{13} - 1 \cdot M_{23} + 1 \cdot M_{33} \\\\&\text{Where } M_{ij} \text{ is the minor of element } (i, j), \text{ i.e., determinant of the } 2 \times 2 \text{ matrix formed by deleting the } i^{th} \text{ row} \\&\text{and } j^{th} \text{ column:} \\\\&M_{33} = \left|\begin{array}{cc}1 & 1 \\x^2 + 2x + 2 & 1\end{array}\right| = (1)(1) - (1)(x^2 + 2x + 2) = 1 - (x^2 + 2x + 2) \\&\qquad = 1 - x^2 - 2x - 2 = -x^2 - 2x - 1 \\\\&\text{Thus:} \\\\&\Delta = - (1 \cdot \left|\begin{array}{cc}1 & 1 \\2 & 1\end{array}\right|) + 1 \cdot \left|\begin{array}{cc}1 & 1 \\x^2 + 2x + 2 & 1\end{array}\right|) \\&\Delta = - (1 \cdot 1 - 1 \cdot 2) + (1 \cdot 1 - 1 \cdot (x^2 + 2x + 2)) \\&\Delta = - (1 - 2) + (1 - x^2 - 2x - 2) \\&\Delta = - (-1) + (-x^2 - 2x - 1) \\&\Delta = 1 - x^2 - 2x - 1 \\&\Delta = -x^2 - 2x \\\\&\text{Set determinant to zero:} \\\\&-x^2 - 2x = 0 \implies x^2 + 2x = 0 \implies x(x + 2) = 0 \implies x = 0 \text{ or } x = -2\end{aligned}​​

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