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NDA 2015
Exam
NDA
Year
2015
Subject
Mathematics
Chapter
Matrices

NDA 2015 · Mathematics · Matrices

If X=[ array ll 3 & -4 1 & -1 array ], B=[ array cc 5 & 2 -2 & 1...

If \quad X=\left[3−41−1\begin{array}{ll}3 & -4 \\ 1 & -1\end{array}\right], B=\left[52−21\begin{array}{cc}5 & 2 \\ -2 & 1\end{array}\right] and A=\left[pqrs\begin{array}{ll}p & q \\ r & s\end{array}\right] satisfy the equation AX=BA X=B, then the matrix AA is equal to

Options

  1. A\left[−7261−5\begin{array}{cc}-7 & 26 \\ 1 & -5\end{array}\right]
  2. B\left[726417\begin{array}{ll}7 & 26 \\ 4 & 17\end{array}\right]
  3. C\left[−7−42613\begin{array}{cc}-7 & -4 \\ 26 & 13\end{array}\right]
  4. D\left[−726−623\begin{array}{ll}-7 & 26 \\ -6 & 23\end{array}\right]

Correct answer

A. \left[−7261−5\begin{array}{cc}-7 & 26 \\ 1 & -5\end{array}\right]

Step-by-step solution

\mathrm{X}=\left[3−41−1\begin{array}{cc}3 & -4 \\ 1 & -1\end{array}\right], \mathrm{B}=\left[52−21\begin{array}{cc}5 & 2 \\ -2 & 1\end{array}\right] and \mathrm{A}=\left[pqrs\begin{array}{ll}\mathrm{p} & \mathrm{q} \\ \mathrm{r} & \mathrm{s}\end{array}\right] Now, \mathrm{AX}=\mathrm{B}\therefore \quad\left[pqrs\begin{array}{ll} \mathrm{p} & \mathrm{q} \\ \mathrm{r} & \mathrm{ s} \end{array}\right]\left[3−41−1\begin{array}{cc} 3 & -4 \\ 1 & -1 \end{array}\right]=\left[52−21\begin{array}{cc} 5 & 2 \\ -2 & 1 \end{array}\right]\Rightarrow\left[3p+q−4p−q3r+s−4r−s\begin{array}{cc}3 p+q & -4 p-q \\ 3 r+s & -4 r-s\end{array}\right]=\left[52−21\begin{array}{cc}5 & 2 \\ -2 & 1\end{array}\right]3p+q=5...(i)−4p−q=2...(ii)3r+s=−2...(iii)−4r−s=1...(iv)\begin{aligned} & 3 p+q=5 ...(i)\\ & -4 p-q=2 ...(ii)\\ & 3 r+s=-2 ...(iii)\\ & -4 r-s=1...(iv) \end{aligned}Fromequations(i)and(ii),wegetFrom equations (i) and (ii), we get\begin{array}{ll} & -p=7 \\ \therefore \quad & p=-7 \\ \Rightarrow \quad q=5-3(-7) \\ & q=26 \end{array}Fromequations(iii)and(iv),−r=−1From equations (iii) and (iv), -r=-1\begin{aligned}& \therefore \quad \mathrm{r}=1 \\ \text{ and } \Rightarrow \quad \mathrm{ s}=-2-3=-5 \\ \text{ and } \therefore \quad \mathrm{ s}=-5 \end{aligned}$ Hence, $A=\left[\begin{array}{ll}\mathrm{p}& \mathrm{q}\\ \mathrm{r}& \mathrm{s}\end{array}\right]=\left[\begin{array}{cc}-7 & 26 \\ 1 & -5\end{array}$\right]\therefore \quad Option (A) is correct.
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