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Question

Let M=āŽ”āŽ¢āŽ£01a1233b1āŽ¤āŽ„āŽ¦ and adj M=āŽ”āŽ¢āŽ£āˆ’11āˆ’18āˆ’62āˆ’53āˆ’1āŽ¤āŽ„āŽ¦ where a,b are real numbers. Which of the following option is/are correct?

A
a+b=3
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B

(adj M)1+adj M1=M
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C
If Mαβγ=123,then αβ+γ=3
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D
det(adj M2)=81
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Solution

The correct option is C If Mαβγ=123,then αβ+γ=3
(adj M)11=23b1=1
23b=1 b=1
(adj M)22=0a31=6
3a=6 a=2
so, a+b=2+1=3

Therefore,M=012123311
det M=0+1(19)+2(16)=2
|adj (M2)|=|M2|2=|M|4=(2)4=16

M1=adj Mdet M=adj M2
so, (adj M)1+adj M1
(M1|M|)1+(M1)1|M1|
(M1)1|M|+M|M|
M2+M2=M
(adj M)1+adj M1=M

If Mαβγ=123
As, MX=BX=M1B=adj M|M|.B
=12111862531123=111

X=111=αβγ
(α,β,γ)=(1,1,1)
Hence, (αβ+γ)=3

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