The cost (in dollars) of producing $x$ units of a certain com-
modity is $C(x)=5000+10 x+0.05 x^{2} .$
(a) Find the average rate of change of $C$ with respect to $x$ when the production level is changed
(i) from $x=100$ to $x=105$
(ii) from $x=100$ to $x=101$
(b) Find the instantaneous rate of change of $C$ with respect
to $x$ when $x=100$ . (This is called the marginal cost. Its
significance will be explained in Section $2.7 . )$