A Matrix is an array of numbers:
To multiply a matrix by a single number is easy:
We call the number ("2" in this case) a scalar, so this is called "scalar multiplication".
The "Dot Product" is where we multiply matching members, then sum up:
(1, 2, 3) • (7, 9, 11) = 1×7 + 2×9 + 3×11
= 58
We match the 1st members (1 and 7), multiply them, likewise for the 2nd members (2 and 9) and the 3rd members (3 and 11), and finally sum them up.
Want to see another example? Here it is for the 1st row and 2nd column:
We can do the same thing for the 2nd row and 1st column:
(4, 5, 6) • (7, 9, 11) = 4×7 + 5×9 + 6×11
= 139
And for the 2nd row and 2nd column:
(4, 5, 6) • (8, 10, 12) = 4×8 + 5×10 + 6×12
= 154
And we get:
