Exercise 2.9: The 
width of an interval
is half of the difference between its upper and lower bounds.  The width is a
measure of the uncertainty of the number specified by the interval.  For some
arithmetic operations the width of the result of combining two intervals is a
function only of the widths of the argument intervals, whereas for others the
width of the combination is not a function of the widths of the argument
intervals.  Show that the width of the sum (or difference) of two intervals is
a function only of the widths of the intervals being added (or subtracted).
Give examples to show that this is not true for multiplication or division.