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If A is square matrix such that A2 = A, then (I + A)3 – 7A is equal to (A) A (B) I – A (C) I (D) 3A |
Answer» Answer is (C) A2 = A (I + A)3 – 7A = (I + A) (I+A)2 – 7A = (I + A) {(I + A) (I+A)} -7A = (I + A) (I2 + 2A + A2) – 7A [v IA = AI = A] = (I + A) (I+ 2A + A) – 7A (A2= A) = (I + A) (I + 3A) – 7A = I2 + AI + 3IA + 3A2 – 7A = I + A + 3A + 3A – 7A [∵ A2 = A & IA = AI = A] =1 |
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