First version of lab_matrices

matrices.md

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+# Matrix multiplication 
+
+Here are the matrices from the lab. They are written as lists of lists, 
+which is a good way to write your answers too.
+
+  a = [[1, 2, 3]] 
+  B = [[6, 1],
+       [1, 4]]
+  C = [[1, 0, 0],
+       [0, 1, 0],
+       [0, 0, 1]]
+  D = [[1, 1],
+       [2, 2],
+       [3, 3]]
+  E = [[1, 2],
+       [1, 2],
+       [1, 2]]
+
+## Exercises
+
+Perform each of the following matrix multiplications if it is allowed.
+If it's not allowed, explain why not.
+
+1. BE
+2. EB
+3. BB
+4. DD
+5. CD
+6. aC