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Matrices and Determinants question

2023 · 8 Apr · Shift 2 · Q31
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Matrices and Determinants question

2023 · 8 Apr · Shift 2 · Q31

JEE MainMathematicsMatrices and DeterminantsMCQ+4 / −1
If A=[15λ10],A−1=αA+βIA=\left[\begin{array}{cc}1 & 5 \\ \lambda & 10\end{array}\right], \mathrm{A}^{-1}=\alpha \mathrm{A}+\beta \mathrm{I}A=[1λ​510​],A−1=αA+βI and α+β=−2\alpha+\beta=-2α+β=−2, then 4α2+β2+λ24 \alpha^{2}+\beta^{2}+\lambda^{2}4α2+β2+λ2 is equal to :
  1. A
    12
  2. B
    10
  3. C
    19
  4. D
    14
View written solutionFree

Correct answer: D

  1. Write the matrix and use the given relation

Given

A=[15λ10],A−1=αA+βIA=\begin{bmatrix}1&5\\ \lambda&10\end{bmatrix}, \qquad A^{-1}=\alpha A+\beta IA=[1λ​510​],A−1=αA+βI

and

α+β=−2.\alpha+\beta=-2.α+β=−2.

We use the identity

AA−1=I.AA^{-1}=I.AA−1=I.

So,

A(αA+βI)=I  ⟹  αA2+βA=I.A(\alpha A+\beta I)=I \implies \alpha A^2+\beta A=I.A(αA+βI)=I⟹αA2+βA=I.
  1. Compute A2A^2A2
A2=[15λ10][15λ10]=[1+5λ5+50λ+10λ5λ+100]=[1+5λ5511λ5λ+100].A^2=\begin{bmatrix}1&5\\ \lambda&10\end{bmatrix} \begin{bmatrix}1&5\\ \lambda&10\end{bmatrix} = \begin{bmatrix} 1+5\lambda & 5+50\\ \lambda+10\lambda & 5\lambda+100 \end{bmatrix} = \begin{bmatrix} 1+5\lambda & 55\\ 11\lambda & 5\lambda+100 \end{bmatrix}.A2=[1λ​510​][1λ​510​]=[1+5λλ+10λ​5+505λ+100​]=[1+5λ11λ​555λ+100​].

Hence,

αA2+βA=[α(1+5λ)+β55α+5β11αλ+βλα(5λ+100)+10β].\alpha A^2+\beta A = \begin{bmatrix} \alpha(1+5\lambda)+\beta & 55\alpha+5\beta\\ 11\alpha\lambda+\beta\lambda & \alpha(5\lambda+100)+10\beta \end{bmatrix}.αA2+βA=[α(1+5λ)+β11αλ+βλ​55α+5βα(5λ+100)+10β​].

Since this equals I=[1001]I=\begin{bmatrix}1&0\\0&1\end{bmatrix}I=[10​01​], compare entries.

  1. Use off-diagonal entries

From the (1,2)(1,2)(1,2) entry:

55α+5β=0  ⟹  11α+β=0.55\alpha+5\beta=0 \implies 11\alpha+\beta=0.55α+5β=0⟹11α+β=0.

Given also

α+β=−2.\alpha+\beta=-2.α+β=−2.

Subtracting,

(11α+β)−(α+β)=0−(−2)(11\alpha+\beta)-(\alpha+\beta)=0-(-2)(11α+β)−(α+β)=0−(−2) 10α=2  ⟹  α=15.10\alpha=2 \implies \alpha=\frac15.10α=2⟹α=51​.

Then

β=−2−15=−115.\beta=-2-\frac15=-\frac{11}{5}.β=−2−51​=−511​.
  1. Find λ\lambdaλ

From the (2,1)(2,1)(2,1) entry:

11αλ+βλ=0  ⟹  λ(11α+β)=0.11\alpha\lambda+\beta\lambda=0 \implies \lambda(11\alpha+\beta)=0.11αλ+βλ=0⟹λ(11α+β)=0.

But already 11α+β=011\alpha+\beta=011α+β=0, so this gives no new information.

Now use the (1,1)(1,1)(1,1) entry:

α(1+5λ)+β=1.\alpha(1+5\lambda)+\beta=1.α(1+5λ)+β=1.

Substitute α=15\alpha=\frac15α=51​, β=−115\beta=-\frac{11}{5}β=−511​:

15(1+5λ)−115=1.\frac15(1+5\lambda)-\frac{11}{5}=1.51​(1+5λ)−511​=1. 1+5λ−115=1  ⟹  5λ−105=1\frac{1+5\lambda-11}{5}=1 \implies \frac{5\lambda-10}{5}=151+5λ−11​=1⟹55λ−10​=1 λ−2=1  ⟹  λ=3.\lambda-2=1 \implies \lambda=3.λ−2=1⟹λ=3.
  1. Compute the required expression
4α2+β2+λ2=4(15)2+(−115)2+324\alpha^2+\beta^2+\lambda^2 =4\left(\frac15\right)^2+\left(-\frac{11}{5}\right)^2+3^24α2+β2+λ2=4(51​)2+(−511​)2+32 =4⋅125+12125+9=4+12125+9=12525+9=5+9=14.=4\cdot \frac1{25}+\frac{121}{25}+9 =\frac{4+121}{25}+9 =\frac{125}{25}+9 =5+9=14.=4⋅251​+25121​+9=254+121​+9=25125​+9=5+9=14.
  1. Final answer
14\boxed{14}14​

So the correct option is D.

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