Q50 / 75 · NTA Abhyas (JEE Main) 2020 · Mathematics · Matrices

Let M and N are two non singular matrices of order 3 with real entries such that adjM=2N and adjN=M. If MN=λΙ, then the value of λ is equal to (where, adjX represents the adjoint matrix of matrix X and I represents an identity matrix)

Correct answer: 2

Solution

adjM=2N⇒adjM=2N⇒M2=8N …. 1
adjN=M⇒adjN=M⇒N2=M …. 2
From 1 and 2, we get,

N=2,M=4
Now, MN=adjNN=NΙ=2Ι⇒λ=2

Hence, the correct answer is 2.

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