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NCERT Solutions for class 12 Maths | Chapter 3 - Matrices

(1) If A is a m × n matrix such that AB and BA are both defined, then B is an
[A] m × n matrix
[B] n × m matrix
[C] n × n matrix
[D] m × m matrix
Answer: n × n matrix
(2) If AB = A and BA = B, then
[A] B = 1
[B] A = I
[C] A² = A
[D] B² = I
Answer: A² = A

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(3) A square matrix A = [aij]n×n is called a lower triangular matrix if aij = 0 for
[A] i = j
[B] i < j
[C] i > j
[D] None of these
Answer: i < j
(4) The number of all possible matrices of order 3 × 3 with each entry 0 or 1 is
[A] 18
[B] 512
[C] 81
[D] None of these
Answer: 512
(5) For any two matrices A and B, we have
[A] AB = BA
[B] AB ≠ BA
[C] AB = 0
[D] None of these
Answer: None of these
(6) If A is a square matrix such that A² = I, then (A – I)³ + (A + I)³ – 7A is equal to
[A] A
[B] I – A
[C] I + A
[D] 3 A
Answer: A
(7) If A is matrix of order m × n and B is a matrix such that AB’ and B’A are both defined, then order of matrix B is
[A] m × m
[B] n × n
[C] n × m
[D] m × n
Answer: m × n
(8) If A and B are two matrices of the order 3 × m and 3 × n, respectively, and m = n, then the order of matrix (5A – 2B) is
[A] m × 3
[B] 3 × 3
[C] m × n
[D] 3 × n
Answer: 3 × n
(9) For any unit matrix I
[A] I² = I
[B] |I| = 0
[C] |I| = 2
[D] |I| = 5
Answer: I² = I
(10) A matrix A = [aij]m×n is said to be skew symmetric if
[A] aij = 0
[B] aij = aji
[C] aij = -aji
[D] aij = 1
Answer: aij = aji
(11) If A and B are square matrices then (AB)’ =
[A] B’A’
[B] A’B’
[C] AB’
[D] A’B’
Answer: B’A’
(12) Let A be a non-singular matrix of the order 2 × 2 then |A-1|=
[A] |A|
[B] 1/|A|
[C] 0
[D] 1
Answer: 1/|A|
(13) If A = then
[A] |A| = 0
[B] A-1 exists
[C] A-1 does not exist
[D] None of these
Answer: A-1 exists
(14) A² – A + I = 0 then the inverse of A
[A] A
[B] A + I
[C] I – A
[D] A – I
Answer: I – A
(15) If a matrix is both symmetric matrix and skew symmetric matrix then
[A] A is a diagonal matrix
[B] A is zero matrix
[C] A is scalar matrix
[D] None of these
Answer: A is zero matrix
(16) Total number of possible matrices of order 3 × 3 with each entry 2 or 0 is
[A] 9
[B] 27
[C] 81
[D] 512
Answer: 512
(17) If A and B are matrices of same order, then (AB’ – BA’) is a
[A] skew symmetric matrix
[B] null matrix
[C] symmetric matrix
[D] unit matrix
Answer: skew symmetric matrix
(18) A square matrix A = [aij]n×n is called a diagonal matrix if aij = 0 for
[A] i = j
[B] i < j
[C] i > j
[D] i ≠ j
Answer: i ≠ j
(19) If A and B are 2 × 2 matrices, then which of the following is true?
[A] (A + B)² = A² + B² + 2AB
[B] (A – B)² = A² + B² – 2AB
[C] (A – B)(A + B) = A² + AB – BA – B²
[D] (A + B) (A – B) = A² – B²
Answer: (A – B)(A + B) = A² + AB – BA – B²
(20) A = [aij]m×n is a square matrix if
[A] m = n
[B] m < n
[C] m > n
[D] None of these
Answer: m = n

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