# Show that satisfies the equation A2 – 3A – 7I = 0 and hence find A–1.

We have the matrix A, such that (i). We need to show that the matrix A satisfies the equation A2 – 3A – 7I = 0.

(ii). Also, we need to find A-1.

(i). Take L.H.S: A2 – 3A – 7I

First, compute A2.

A2 = A.A In order to multiply two matrices, A and B, the number of columns in A must equal the number of rows in B. Thus, if A is an m x n matrix and B is an r x s matrix, n = r.

Multiply 1st row of matrix A by matching members of 1st column of matrix A, then sum them up.

(5, 3).(5, -1) = (5 × 5) + (3 × -1)

(5, 3).(5, -1) = 25 + (-3)

(5, 3).(5, -1) = 25 – 3

(5, 3).(5, -1) = 22 Multiply 1st row of matrix A by matching members of 2nd column of matrix A, then sum them up.

(5, 3).(3, -2) = (5 × 3) + (3 × -2)

(5, 3).(3, -2) = 15 + (-6)

(5, 3).(3, -2) = 15 – 6

(5, 3).(3, -2) = 9 Multiply 2nd row of matrix A by matching members of 1st column of matrix A, then sum them up.

(-1, -2).(5, -1) = (-1 × 5) + (-2 × -1)

(-1, -2).(5, -1) = -5 + 2

(-1, -2).(5, -1) = -3 Multiply 2nd row of matrix A by matching members of 2nd column of matrix A, then sum them up.

(-1, -2).(3, -2) = (-1 × 3) + (-2 × -2)

(-1, -2).(3, -2) = -3 + 4

(-1, -2).(3, -2) = 1  Substitute values of A2 and A in A2 – 3A – 7I. Also, since matrix A is of the order 2 × 2, then I will be the identity matrix of order 2 × 2 such that,      Clearly,

L.H.S = R.H.S

Thus, we have shown that matrix A satisfy A2 – 3A – 7I = 0.

(ii). Now, let us find A-1.

We know that, inverse of matrix A is A-1 is true only when

A × A-1 = A-1 × A = I

Where, I = Identity matrix

We have,

A2 – 3A – 7I = 0

Multiply A-1 on both sides, we get

A-1(A2 – 3A – 7I) = A-1 × 0

A-1.A2 – A-1.3A – A-1.7I = 0

A-1.A.A – 3A-1.A – 7A-1.I = 0

(A-1A)A – 3(A-1A) – 7(A-1I) = 0

And as A-1A = I and A-1I = A-1

IA – 3I – 7A-1 = 0

Since, IA = A

A – 3I – 7A-1 = 0

7A-1 = A – 3I  [ ]   Thus, .

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