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JEE Advanced · Mathematics · 18. Matrices

Let P=3-1-220α3-50 , where αR, Suppose Q=qij is a matrix such thatPQ = kI, where kR, k0 andI is the identity matrix of order 3. If q23= -k8 and detQ=k22 then 

  1. A α=0, k=8
  2. B 4α-k+8=0
  3. C detPadjQ=29
  4. D detQ(adjP)=213
Verified Solution

Answer & Solution

Correct Answer

(C) detPadjQ=29

Step-by-step Solution

Detailed explanation

PQ=kI
PQ=k3     ( as order is 3 )
P=2k 0    P is an invertible matrix
PQ=kI
Q=kP-1 I =K1P adj P
Q=adj.P2
q23= -k8( cofactor P32= -3α+4)
12α+16=K &adjP2=P32 2T-q23
undefined ...... (i)
Also P=2k k=10+6 α       ......(ii)
Solving these two 12α+16=10+6αα=-1 & K=4
4α-k+8=0   ( 4(1)(4)+8=044+8=08+8=0 )
And detPadj (Q=P adj (Q)=2k. k222=k52=29
Also det Q adj P = Q adj P = k 2 2 2k 2 = 2k 4 = 2 9
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