(i) Determine which of the following matrix products can be found, and find them.

(a) AB

(b) AC

(c) CA

(d) CB

(e) BC (ii) Suppose that you have an N x 1 vector 1, whose every element consists of the number one (1). Now take a set of sample observations on some random variable x, namely x1 , x2 ,....xN, and assemble them into a column vector x. What does the product 1’x look like? The vector 1 is called the summation vector – it does the same thing as the Σ sign.

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A= 2 1 1 105 3 42 1 B= C= 1 2 12 1 0 3 4