# If A = [101023121]and B=[123115247], then find a matrix X such that XA = B. - Mathematics and Statistics

Sum

If A = [(1, 0, 1),(0, 2, 3),(1, 2, 1)] "and B" = [(1, 2, 3),(1, 1, 5),(2, 4, 7)], then find a matrix X such that XA = B.

#### Solution 1

A = [(1, 0, 1),(0, 2, 3),(1, 2, 1)] and B = [(1, 2, 3),(1, 1, 5),(2, 4, 7)]

XA = B

Post multiplying by A–1, we get

XAA–1 = BA–1

∴ X = BA–1         ...(i)

|A| = |(1, 0, 1),(0, 2, 3),(1, 2, 1)|

= 1(2 – 6) – 0 + 1(0 – 2)

= – 4 – 2

= – 6 ≠ 0

∴ A–1 exists.

A11 = (– 1)1+1 M11 = 1 |(2, 3),(2, 1)| = 1(2 – 6) = – 4

A12 = (– 1)1+2 M12 = -1 |(0, 3),(1, 1)| = –1(0 – 3) = 3

A13 = (– 1)1+3 M13 = 1 |(0, 2),(1, 2)| = 1(0 – 2) = – 2

A21 = (– 1)2+1 M21 = -1 |(0, 1),(2, 1)| = –1(0 – 2) = 2

A22 = (– 1)2+2 M22 = 1 |(1, 1),(1, 1)| = 1(1 – 1) = 0

A23 = (– 1)2+3 M23 = -1 |(1, 0),(1, 2)| = –1(2 – 0) = – 2

A31 = (– 1)3+1 M31 = 1 |(0, 1),(2, 3)| = 1(0 – 2) = – 2

A32 = (– 1)3+2 M32 = -1 |(1, 1),(0, 3)| = –1(3 – 0) = – 3

A33 = (– 1)3+3 M33 = 1 |(1, 0),(0, 2)| = 1(2 – 0) =  2

∴ The matrix of the co-factors is

[Aij]3×3 = [("A"_11, "A"_12, "A"_13),("A"_21, "A"_22, "A"_23),("A"_31, "A"_32, "A"_33)]

= [(-4, 3, -2),(2, 0, -2),(-2, -3, 2)]

Now, adj A = ["A"_"ij"]_(3 xx 3)^"T"

= [(-4, 2, -2),(3, 0, -3),(-2, -2, 2)]

∴ A–1 = 1/|"A"| ("adj A")

= -1/6[(-4, 2, -2),(3, 0, -3),(-2, -2, 2)]

X = BA–1             ...[From (i)]

∴ X = [(1, 2, 3),(1, 1, 5),(2, 4, 7)] {(1/6) [(-4, 2, -2),(3, 0, -3),(-2, -2, 2)]}

= (1)/(6) [(1, 2, 3),(1, 1, 5),(2, 4, 7)] [(4, -2, 2),(-3, 0, 3),(2, 2, -2)]

= (1)/(6) [(4 - 6 + 6, -2 + 0 + 6, 2 + 6 - 6),(4 - 3 + 10, -2 + 0 + 10, 2 + 3 - 10),(8 - 12 + 14, -4 + 0 + 14, 4 + 12 - 14)]

∴ X = (1)/(6)[(4, 4, 2),(11, 8, -5),(10, 10, 2)]

#### Solution 2

Consider XA = B

∴ X[(1, 0, 1),(0, 2, 3),(1, 2, 1)] = [(1, 2, 3),(1, 1, 5),(2, 4, 7)]

By C3 – C1, we get

[(1, 0, 0),(0, 2, 3),(1, 2, 0)] = [(1, 2, 2),(1, 1, 4),(2, 4, 5)]

By (1/2)C_2, we get

[(1, 0, 0),(0, 1, 3),(1, 1, 0)] = [(1, 1, 2),(1, 1/2, 4),(2, 2, 5)]

By C3 – 3C2, we get

[(1, 0, 0),(0, 1, 0),(1, 1, -3)] = [(1, 1, -1),(1, 1/2, 5/2),(2, 2, -1)]

By (-1/3)C_3, we get

[(1, 0, 0),(0, 1, 0),(1, 1, 1)] = [(1, 1, 1/3),(1, 1/2, -5/6),(2, 2, 1/3)]

By C1 – C3 and C2 – C3, we get

[(1, 0, 0),(0, 1, 0),(0, 0, 1)] = [(2/3, 2/3, 1/3),(11/6, 4/3, -5/6),(5/3, 5/3, 1/3)]

∴ X = 1/6[(4, 4, 2),(11, 8, -5),(10, 10, 2)]

Is there an error in this question or solution?
Chapter 2: Matrices - Exercise 2.5 [Page 72]

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