if AB=A and BA=B,show that A and B are idempotent
Answers
Solution :
A matrix A is known as idempotent if it yields itself if it is multiplied by itself i.e, if A² = A.
Given,
AB = A
Pre multiply A inverse(A^{-1}) on both sides
A^{-1} A B = A^{-1} A
B = I (identity matrix)
Similarly, A = I
A² = A.A = I.I = I² = I = A
Similarly, B² = B
Therefore, A and B are idempotent matrices.
Step-by-step explanation:
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If AB=A and BA=B, then which of the following is/are true?
THIS QUESTION HAS MULTIPLE CORRECT OPTIONS
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ANSWER
Given, AB=A and BA=B
Take, AB=A
⇒A(BA)=A [ Since, B=BA ]
⇒(AB)A=A
⇒AA=A [ Since, AB=A ]
⇒A
2
=A
Therefore, A is an idempotent matrix.
Take, BA=B
⇒B(AB)=B [ Since, A=AB ]
⇒(BA)B=B
⇒BB=B [ Since, BA=B ]
⇒B
2
=B
Therefore, B is an idempotent matrix.
Now, A=AB
Applying transpose on both sides, we get
A
T
=(AB)
T
⇒A
T
=B
T
.A
T
....(i)
Also, B=BA
Applying transpose on both sides, we get
B
T
=(BA)
T
⇒B
T
=A
T
.B
T
...(ii)
From equation (i) and equation (ii), we get
(A
T
)
2
=A
T
and
(B
T
)
2
+B
T
Therefore, A
T
and B
T
are alos idempotent matrices.
Hence, the correct options are (A),(B) and (C).
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