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NDA 2015
Exam
NDA
Year
2015
Subject
Mathematics
Chapter
Matrices

NDA 2015 · Mathematics · Matrices

Consider the following in respect of two non-singular matrices A...

Consider the following in respect of two non-singular matrices AA and BB of same order: 1. det⁡⁡(A+B)=det⁡⁡A+det⁡⁡B2.\operatorname\det(A+B)=\operatorname\det A+\operatorname\det B 2.\quad(A+B)^{-1}=A^{-1}+B^{-1}Whichoftheaboveis/arecorrect?Which of the above is/are correct?

Options

  1. A1 only
  2. B2 only
  3. CBoth 1 and 2
  4. DNeither 1 nor 2

Correct answer

D. Neither 1 nor 2

Step-by-step solution

Non-singular matrix is a matrix whose determinate Value is non-zero. Let A=\left[1112\begin{array}{ll}1 & 1 \\ 1 & 2\end{array}\right] and B=\left[2112\begin{array}{ll}2 & 1 \\ 1 & 2\end{array}\right] Here, AA and BB are non-singular matrix Now from Statement 1: A+B=[1112]+[2112]=[3224]det⁡(A+B)=12−4=8 Now det (A)=∣1112∣=1 and det⁡(B)=∣2112∣=3\begin{aligned} & A+B=\left[\begin{array}{ll} 1 & 1 \\ 1 & 2 \end{array}\right]+\left[\begin{array}{ll} 2 & 1 \\ 1 & 2 \end{array}\right]=\left[\begin{array}{ll} 3 & 2 \\ 2 & 4 \end{array}\right] \\ & \operatorname{det}(A+B)=12-4=8 \\ & \text{ Now det }(A)=\left|\begin{array}{ll} 1 & 1 \\ 1 & 2 \end{array}\right|=1 \\ & \text{ and } \operatorname{det}(B)=\left|\begin{array}{ll} 2 & 1 \\ 1 & 2 \end{array}\right|=3 \end{aligned} Now, det⁡(A+B)=4≠det⁡(A+B)∴\operatorname{det}(A+B)=4 \neq \operatorname{det}(A+B)\therefore \quad Statement 1 is wrong. Now from Statement 2: (A+B)−1=1∣A+B∣adj⁡(A+B)=18[4−2−23] and A−1=1∣A∣adj⁡(A)=11[2−1−11] and B−1=1∣B∣adj⁡(B)=13[2−1−12] Now, A−1+B−1=[2−1−11]+[23−13−1323]\begin{aligned} & (A+B)^{-1}=\frac{1}{|A+B|} \operatorname{adj}(A+B) \\ & =\frac{1}{8}\left[\begin{array}{cc} 4 & -2 \\ -2 & 3 \end{array}\right] \\ & \text{ and } \mathrm{A}^{-1}=\frac{1}{|\mathrm{ A}|} \operatorname{adj}(\mathrm{A})=\frac{1}{1}\left[\begin{array}{cc} 2 & -1 \\ -1 & 1 \end{array}\right] \\ & \text{ and } B^{-1}=\frac{1}{|B|} \operatorname{adj}(B)=\frac{1}{3}\left[\begin{array}{cc} 2 & -1 \\ -1 & 2 \end{array}\right] \\ & \text{ Now, } A^{-1}+B^{-1}=\left[\begin{array}{cc} 2 & -1 \\ -1 & 1 \end{array}\right]+\left[\begin{array}{cc} \frac{2}{3} & \frac{-1}{3} \\ \frac{-1}{3} & \frac{2}{3} \end{array}\right] \end{aligned}=\left[83−43−4353\begin{array}{cc} \frac{8}{3} & \frac{-4}{3} \\ \frac{-4}{3} & \frac{5}{3} \end{array}\right] \neq(A+B)^{-1}\therefore \quadStatement2iswrongStatement 2 is wrong\therefore \quad$ Option (D) is correct.
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