A simple courve that often makes a good model for the variable costs of a company as a

function of the sales level x has the form y $=\backslash$beta $1$x $+\backslash$beta $2$x

$2+\backslash$beta $3$x

$3$

$.$ There is no constant term because fixed

costs are not included.

a$)$ Give the design matrix and the parameter vector for the li