1. Definition and Examples
1. Scalar Multiplication
If `A=[[2,3,1],[0,5,6],[1,1,2]]` then
`2*A=2*[[2,3,1],[0,5,6],[1,1,2]]=[[2*2,2*3,2*1],[2*0,2*5,2*6],[2*1,2*1,2*2]]=[[4,6,2],[0,10,12],[2,2,4]]`
2. Multiplication of matrices
If `A=[[a_11,a_12,a_13],[a_21,a_22,a_23],[a_31,a_32,a_33]]` and `B=[[b_11,b_12,b_13],[b_21,b_22,b_23],[b_31,b_32,b_33]]` then
`A xx B = [[a_11,a_12,a_13],[a_21,a_22,a_23],[a_31,a_32,a_33]] xx [[b_11,b_12,b_13],[b_21,b_22,b_23],[b_31,b_32,b_33]]`
`= [[a_11 b_11 + a_12 b_21 + a_13 b_31, a_11 b_12 + a_12 b_22 + a_13 b_32, a_11 b_13 + a_12 b_23 + a_13 b_33],
[a_21 b_11 + a_22 b_21 + a_23 b_31, a_21 b_12 + a_22 b_22 + a_23 b_32, a_21 b_13 + a_22 b_23 + a_23 b_33],
[a_31 b_11 + a_32 b_21 + a_33 b_31, a_31 b_12 + a_32 b_22 + a_33 b_32, a_31 b_13 + a_32 b_23 + a_33 b_33]
]`
3. Properties of Matrix Multiplication
1. Matrix Multiplication is not Commutative
`AB != BA`
2. Matrix Multiplication is Associative
`A(BC) = (AB)C`
3. Matrix Multiplication is Distributive
`A(B+C) = AB+AC`
4. Matrix Multiplication by Unit matrix
`AI=IA=A`
5. `A * A^-1 = A^-1 * A = I`
Examples
1. Find `A × B` ... `A=[[3,1,1],[-1,2,1],[1,1,1]]`,`B=[[5,0,-2],[7,-6,0],[1,1,2]]`
Solution:
`A×B` | = | | `3` | `1` | `1` | | | `-1` | `2` | `1` | | | `1` | `1` | `1` | |
| × | | `5` | `0` | `-2` | | | `7` | `-6` | `0` | | | `1` | `1` | `2` | |
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= | | `3×5+1×7+1×1` | `3×0+1×-6+1×1` | `3×-2+1×0+1×2` | | | `-1×5+2×7+1×1` | `-1×0+2×-6+1×1` | `-1×-2+2×0+1×2` | | | `1×5+1×7+1×1` | `1×0+1×-6+1×1` | `1×-2+1×0+1×2` | |
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= | | `15+7+1` | `0-6+1` | `-6+0+2` | | | `-5+14+1` | `0-12+1` | `2+0+2` | | | `5+7+1` | `0-6+1` | `-2+0+2` | |
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= | | `23` | `-5` | `-4` | | | `10` | `-11` | `4` | | | `13` | `-5` | `0` | |
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2. Find `A × B` ... `A=[[2,1,1],[-1,2,1],[1,1,1]]`,`B=[[2,3,1],[0,5,6],[1,1,2]]`
Solution:
`A×B` | = | | `2` | `1` | `1` | | | `-1` | `2` | `1` | | | `1` | `1` | `1` | |
| × | | `2` | `3` | `1` | | | `0` | `5` | `6` | | | `1` | `1` | `2` | |
|
= | | `2×2+1×0+1×1` | `2×3+1×5+1×1` | `2×1+1×6+1×2` | | | `-1×2+2×0+1×1` | `-1×3+2×5+1×1` | `-1×1+2×6+1×2` | | | `1×2+1×0+1×1` | `1×3+1×5+1×1` | `1×1+1×6+1×2` | |
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= | | `4+0+1` | `6+5+1` | `2+6+2` | | | `-2+0+1` | `-3+10+1` | `-1+12+2` | | | `2+0+1` | `3+5+1` | `1+6+2` | |
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= | | `5` | `12` | `10` | | | `-1` | `8` | `13` | | | `3` | `9` | `9` | |
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This material is intended as a summary. Use your textbook for detail explanation. Any bug, improvement, feedback then
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