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JEE Advanced 2019Paper 1
Exam
JEE Advanced
Year
2019
Paper
Paper 1
Subject
Mathematics
Chapter
Matrices

JEE Advanced 2019 · Paper 1 · Mathematics · Matrices

Let M = [ cases 0 & 1 & a 1 & 2 & 3 3 & b & 1 cases ] and adjM = [...

Let M = \left[\begin{cases}0 & 1 & a \\ 1 & 2 & 3 \\ 3 & b & 1\end{cases}\right] and adjM = \left[\begin{cases}- 1 & 1 & - 1 \\ 8 & - 6 & 2 \\ - 5 & 3 & - 1\end{cases}\right] where aa and bb are real numbers. Which of the following options is/are correct?

Options

  1. Aa+b=3a + b = 3
  2. Bdet(adjM2)=81det\left(adj{M}^{2}\right) = 81
  3. C(adjM)−1+adjM−1=−M{\left(adjM\right)}^{- 1} + adj{M}^{- 1} = - M
  4. DIf M=M =\left[αβγ\begin{array}{ll}\alpha \\ \beta \\ \gamma\end{array}\right] = \left[123\begin{array}{ll}1 \\ 2 \\ 3\end{array}\right],thenthen\alpha - \beta + \gamma = 3

Correct answer

A. a+b=3a + b = 3

Step-by-step solution

Given, M = \left[\begin{cases}0 & 1 & a \\ 1 & 2 & 3 \\ 3 & b & 1\end{cases}\right] ...(i)
adj\left(M\right) = \left[\begin{cases}- 1 & 1 & - 1 \\ 8 & - 6 & 2 \\ - 5 & 3 & - 1\end{cases}\right] ...(ii)
Option (A)( A ) :
Now from matrix MM
(adjM)11=2−3b{\left(adjM\right)}_{11} = 2 - 3b
As given 2−3b=−1⇒b=12 - 3b = - 1 ⇒ b = 1

Similarly, a=2a = 2

a+b=3a + b = 3
Using the values of aanda andbinmatrixin matrixM ⇒ | M | = - 2<br/><br />| adj\left({M}^{2}\right) | = | {M}^{2}{|}^{2} = | M{|}^{4} = 16<br/>Now,<br />Now,{M}^{- 1} = \dfrac{adjM}{\left|M\right|}<br/><br />adjM = \left|M\right|{M}^{- 1}<br/><br />adj ( M ) = - 2{M}^{- 1}<br/><br />{\left(adj\left(M\right)\right)}^{- 1} = - \dfrac{1}{2}{\left({M}^{- 1}\right)}^{- 1}((KA)−1=1KA−1)\left({\left(KA\right)}^{- 1} = \dfrac{1}{K}{A}^{- 1}\right)
(adj(M))−1=−M2{( adj ( M ) )}^{- 1} = \dfrac{- M}{2}
and adj(M−1)=(M−1)−1∣M−1∣adj ( {M}^{- 1} ) = {( {M}^{- 1} )}^{- 1} | {M}^{- 1} | \left(adjA = \left|A\right|{A}^{1}\right)<br/><br />adj\left({M}^{- 1}\right) = \dfrac{M}{\left|M\right|}(∣A−1∣=1∣A∣)\left(\left|{A}^{- 1}\right| = \dfrac{1}{\left|A\right|}\right)
adj(M−1)=−M2adj\left({M}^{- 1}\right) = - \dfrac{M}{2} ...(iii)
From (iii) and (iv)
(adj(M)−1+(adj(M−1))=−M( adj{\left(M\right)}^{- 1} + \left(adj\left({M}^{- 1}\right)\right) = - M
Option (D)( D ) :
MX = B\left(whenx = \left[αβγ\begin{array}{ll}\alpha \\ \beta \\ \gamma\end{array}\right]andB = \left[123\begin{array}{ll}1 \\ 2 \\ 3\end{array}\right]\right)
X=M−1(B)X = {M}^{- 1} ( B )
X=adj(M)∣M∣BX = \dfrac{adj\left(M\right)}{\left|M\right|}B
\left[αβγ\begin{array}{ll}\alpha \\ \beta \\ \gamma\end{array}\right] = - \dfrac{1}{2}\left[{−11−18−62−53−1\begin{cases}- 1 & 1 & - 1 \\ 8 & - 6 & 2 \\ - 5 & 3 & - 1\end{cases}\right]\left[123\begin{array}{ll}1 \\ 2 \\ 3\end{array}\right]<br/><br />= - \dfrac{1}{2}\left[−22−2\begin{array}{ll}- 2 \\ 2 \\ - 2\end{array}\right]<br />\left[αβγ\begin{array}{ll}\alpha \\ \beta \\ \gamma\end{array}\right] = \left[1−11\begin{array}{ll}1 \\ - 1 \\ 1\end{array}\right]
∵α=1β=−1γ=1∵ \alpha = 1\beta = - 1\gamma = 1
α−β+γ=3\alpha - \beta + \gamma = 3

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