Let M = a i j ,   i ,   j ∈ { 1 , 2 , 3 } , be the 3 × 3 matrix such that a i j = 1 if…

Let M=aij, i, j{1,2,3}, be the 3×3 matrix such that aij=1 if j+1 is divisible by i, otherwise aij=0. Then which of the following statements is(are) true?
  1. M is invertible
  2. There exists a nonzero column matrix a1a2a3 such that Ma1a2a3=-a1-a2-a3
  3. The set X3:MX=0{0}, where 0=000
  4. The matrix (M-2 I) is invertible, where I is the 3×3 identity matrix

Solution

Given,

M=aij, i, j{1,2,3}

aij=1 if j+1 is divisible by i

otherwise aij=0

So, a11=1, a12=1, a13=1, a21=1, a22=0......,a33=0

So, M=1    1    11    0    10    1    0

|M|=1(1)1(1)=1+1=0

Hence, M is not invertible

Now solving option B we get,

111101010a1a2a3=-a1-a2-a3

a1+a2+a3a1+a3a2=-a1-a2-a3

Now on comparing we get,

a1+a2+a3=-a1, a1+a3=-a2 & a2=-a3

Now on solving we get, a1=0 & a2+a3=0

So, there will be infinite possibility for a2 & a3

There exist a column matrix (infinite possibilities)

Now solving option C we get,
111101010xyz=000

x+y+zx+zy=000

Now on comparing we get,

x+y+z=0x+z=0y=0

Yes it is possible

Now solving option D we get,

|M2I|=111121012

|M2I|=13121=3+3=0

Hence, it is not invertible.

Asked in: JEE Advanced 2023 (Paper 2)

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