Let $R=\begin{bmatrix} a & 3 & b \\ c & 2 & d \\ 0 & 5 & 0 \end{bmatrix}$ : a, b, c, d $\in \{0, 3, 5, 7, 11…

Let $R=\begin{bmatrix} a & 3 & b \\ c & 2 & d \\ 0 & 5 & 0 \end{bmatrix}$ : a, b, c, d $\in \{0, 3, 5, 7, 11, 13, 17, 19\}$. Then the number of invertible matrices in $R$ is

Solution

$R=\begin{bmatrix} a & 3 & b \\ c & 2 & d \\ 0 & 5 & 0 \end{bmatrix}$ $\Rightarrow |R|=-5 \begin{vmatrix} a & b \\ c & d \end{vmatrix}$

We know that, for invertible matrices R0

Now |R| can be zero in following cases:

(i) Two of a, b, c, d are zeroes which can be (a and b),(b and d),(d and c) or (c and a)

4×72 ways =196

(ii) Any three of a, b, c, d are zeroes

C34×7=28

(iii) All four of a, b, c, d are zeroes

1

(iv) All four of a, b, c, d are non-zero but same number

7

(v) When two are alike and 2 other are alike (non-zero)

7C2×2×2=84

Number of invertible matrices

=84-196-28-1-7-84=3780

Asked in: JEE Advanced 2023 (Paper 2)

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