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Determinants · Question 3

JEE Main 2026 (Online) 6th April Morning ShiftNumericalMedium📈 Topic in 42% papers+4
1

Let A= bmatrix-1 & 1 & -1\ 1 & 0 & 1\ 0 & 0 & 1 bmatrix satisfy A2+ (adj (adj(A) ) )+ (adj(A) (adj (adj(A) ) ) )= bmatrix2 & -2 & 2\ -2 & 0 & -1\ 0 & 0 & -1 bmatrix for some , R. Then ( - )2 is equal to .

Answer

4

Solution

Expanding along R3 : |A|=1 vmatrix-1 & 1\ 1 & 0 vmatrix=-1

For a 3 3 matrix : adj(adj A)=|A|3-2A=-A

adj(A) (adj(adj A) )=-adj(A) A=-|A| I=I

A2= bmatrix-1 & 1 & -1\ 1 & 0 & 1\ 0 & 0 & 1 bmatrix bmatrix-1 & 1 & -1\ 1 & 0 & 1\ 0 & 0 & 1 bmatrix= bmatrix2 & -1 & 1\ -1 & 1 & 0\ 0 & 0 & 1 bmatrix

The given equation is A2- A+ I= bmatrix2 & -2 & 2\ -2 & 0 & -1\ 0 & 0 & -1 bmatrix

Entry (1, 2) : -1- =-2 =1

Entry (3, 3) : 1- + =-1 =-1

(All other entries also agree, e.g. (1, 1) : 2+1-1=2, (2, 3) : 0-1=-1)

( - )2=(1+1)2=4

Answer: 4

JEE Main 2026 (Online) 6th April Morning Shift

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