Question

Compute the products AB and BA whichever exists in each of the following cases:

 [a, b]`[[c],[d]]`+ [a, b, c, d] `[[a],[b],[c],[d]]`

Answer 1

[a    b] `[[c],[d]]`+[a     b     c    d]  `[[a],[b],[c],[d]]`

`⇒ [ac+ bd]+[a^2+b^2+c^2+d^2] `

`[a^2+b^2+c^2+d^2+ac+bd]`