Question

If A = `[(1, 3, 2),(2, 0, -1),(1, 2, 3)]`, then show that A satisfies the equation A³ – 4A² – 3A + 11I = O.

Answer 1

A² = A × A = `[(1, 3, 2),(2, 0, -1),(1, 2, 3)] xx [(1, 3, 2),(2, 0, -1),(1, 2, 3)]`

= `[(1 + 6 + 2, 3 + 0 + 4, 2 - 3 + 6),(2+ 0 - 1, 6 + 0 - 2, 4 + 0 - 3),(1 + 4 + 3, 3 + 0 + 6, 2 - 2 + 9)]`

= `[(9, 7, 5),(1, 4, 1),(8, 9, 9)]`

and A³ = A² × A = `[(9, 7, 5),(1, 4, 1),(8, 9, 9)] xx [(1, 3, 2),(2, 0, -1),(1, 2, 3)]`

= `[(9 + 14 + 5, 27 + 0 + 10, 18 - 7 + 15),(1 + 8 + 1, 3 + 0 + 2, 2 - 4 + 3),(8 + 18 + 9, 24 + 0 + 18, 16 - 9 + 27)]`

= `[(28, 37, 26),(10, 5, 1),(35, 42, 34)]`

Now A³ – 4A² – 3A + 11(I)

= `[(28, 37, 36),(10, 5, 1),(35, 42, 34)] -4[(9, 7, 5),(1, 4, 1),(8, 9, 9)] -3[(1, 3, 2),(2, 0, -1),(1, 2, 3)] +11[(1, 0, 0),(0, 1, 0),(0, 0, 1)]`

= `[(28 - 36 - 3 + 11, 37 - 28 - 9 + 0, 26 - 20 - 6 + 0),(10 - 4 - 6 + 0, 5 - 16 + 0 + 11, 1 - 4 + 3 + 0),(35 - 32 - 3 + 0, 42 - 36 - 6 + 0, 34 + 36 - 9 + 11)]`

= `[(0, 0, 0),(0, 0, 0),(0, 0, 0)]` = O