1.

If A is a square matrix such that A2 = A, then (I + A)3 – 7A is equal to :A. A B. I – A C. I D. 3A

Answer»

C. I

(I + A)3 = I3 + A3 + 3A2I + 3AI2 

(Using the identity of (a + b)3 = a3 + b3 + 3ab (a + b)) 

(I + A)3 = I + A2(A) + 3AI + 3A 

[I stands for Identity Matrix] 

(I + A)3 = I + A2 + 3A + 3A 

(I + A)3 = 7A + I (I + A)3 – 7A 

7A + I – 7A 

= I

Option (C) is the answer.



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