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Matrix multiplication examples | 2/8 Session Links |
Here are four examples of matrix products. In each case, the left side of the equality contains the two matrices that are being multiplied, and the right side shows the result. The Excel worksheet function that produces the matrix product of two matrices is mmult.
Of the four examples shown here, only one is a legal product if the order of the two matrix factors on the left side of the equality is reversed. The one that’s legal in reverse order is the lower left product. Try reversing the order, and notice that the result of the reversed-order product isn’t equal to the result shown. This is an illustration of the fact that matrix multiplication isn’t commutative.
Sometimes we want to form the product of three or more matrices. Using mmult, we must do this by first forming the product of two of them, then multiplying the result of that by the third, and so on, maintaining the correct order, of course. It doesn’t matter how you pair them, as long as you preserve the order, because matrix multiplication is associative.
In your add-in for this course, you’ll find a function MMMult, which can handle up to ten matrix factors.
Last Modified: Wednesday, 27-Apr-2016 04:15:26 EDT
For many of you, matrix multiplication and array arithmetic are new ideas. It’s easy to get lost in the details of how they work and then forget about why we use them.
To keep a clear view of the forest and avoid focusing only on the trees, remember why we use matrix multiplication and array arithmetic. Briefly, we use them because we find that it’s very often helpful to decompose a problem into parts (analysis), then do calculations on the parts, and finally reassemble the final solution from the results of those partial calculations (synthesis).
Matrix multiplication and array arithmetic provide us with very convenient methods for performing those intermediate calculations on the parts. They’re the tools that make analysis and synthesis so powerful.