Q169 / 270 · NDA 2017 (Phase 2) · Mathematics

If $a, b, c$ are non-zero real numbers, then the inverse of the $\operatorname{matrix} A=\left[\begin{array}{lll}\mathrm{a} & 0 & 0 \\ 0 & \mathrm{~b} & 0 \\ 0 & 0 & \mathrm{c}\end{array}\right]$ is equal to

  • Correct answer
    A
    $\left[\begin{array}{lll}\mathrm{a}^{-1} & 0 & 0 \\ 0 & \mathrm{~b}^{-1} & 0 \\ 0 & 0 & \mathrm{c}^{-1}\end{array}\right]$
  • B
    $\frac{1}{\mathrm{abc}}\left[\begin{array}{lll}\mathrm{a}^{-1} & 0 & 0 \\ 0 & \mathrm{~b}^{-1} & 0 \\ 0 & 0 & \mathrm{c}^{-1}\end{array}\right]$
  • C
    $\frac{1}{\mathrm{abc}}\left[\begin{array}{lll}1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1\end{array}\right]$
  • D
    $\frac{1}{\mathrm{abc}}\left[\begin{array}{lll}\mathrm{a} & 0 & 0 \\ 0 & \mathrm{~b} & 0 \\ 0 & 0 & \mathrm{c}\end{array}\right]$

Solution

$\mathrm{A}=\left[\begin{array}{lll}\mathrm{a} & 0 & 0 \\ 0 & \mathrm{~b} & 0 \\ 0 & 0 & \mathrm{c}\end{array}\right]$ $\mathrm{A}^{-1}=\left[\begin{array}{ccc}\frac{1}{\mathrm{a}} & 0 & 0 \\ 0 & \frac{1}{\mathrm{~b}} & 0 \\ 0 & 0 & \frac{1}{\mathrm{c}}\end{array}\right]=\left[\begin{array}{ccc}\mathrm{a}^{-1} & 0 & 0 \\ 0 & \mathrm{~b}^{-1} & 0 \\ 0 & 0 & \mathrm{c}^{-1}\end{array}\right]$

Hence, the correct answer is option: 'A'.

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