Working with arrays

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• Subject: [mg131190] Working with arrays
• From: amannucci <Anthony.J.Mannucci at jpl.nasa.gov>
• Date: Sun, 16 Jun 2013 05:19:15 -0400 (EDT)
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```I thought I understood variables. This sequence completely mystifies me:
Clear[lA, xtest]
lA = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
xtest = {4, 5, 6};
lA\[CenterDot]xtest (* First case *)
{{a, b}, {c, d}} . {x, y} (* Second case *)

The output is:

{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}\[CenterDot]{4, 5, 6}

{a x + b y, c x + d y}

How do I force matrix multiplication to actually occur, as in the second answer? Why does Mathematica do the matrix multiply in the second case but not the first?

Thanks for any help.

-Tony

```

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