A shipping company charges for packages according to weight \(w\) in pounds:
- Up to and including \(2\,\text{lb}\): \(\$4.95\)
- More than \(2\,\text{lb}\) and up to and including \(5\,\text{lb}\): \(\$6.95\)
- More than \(5\,\text{lb}\) and up to and including \(10\,\text{lb}\): \(\$10.45\)
- More than \(10\,\text{lb}\) and up to and including \(31.5\,\text{lb}\): \(\$18.45\)
a) Find the shipping cost for packages weighing \(1.8\,\text{lb}\), \(5.0\,\text{lb}\), and \(5.1\,\text{lb}\).
b) Write the cost function \(K(w)\) as a piecewise function.
c) Determine whether \(K\) is continuous at \(w=5\). Justify your answer using the left-hand and right-hand limits.
Hints
- Pay attention to whether each boundary value is included in the lower or upper interval.
- Use piecewise notation to show a different constant value on each weight interval.
- Continuity requires the two one-sided limits to be equal.
Solution
1. The package weighing \(1.8\,\text{lb}\) is in the first interval, so its cost is \(\$4.95\). The package weighing \(5.0\,\text{lb}\) is included in the second interval, so its cost is \(\$6.95\). The package weighing \(5.1\,\text{lb}\) is in the third interval, so its cost is \(\$10.45\).
2. The cost function is \(K(w)=\begin{cases}\$4.95&\text{if }0<w\le2\\\$6.95&\text{if }2<w\le5\\\$10.45&\text{if }5<w\le10\\\$18.45&\text{if }10<w\le31.5\end{cases}\).
3. At \(w=5\), \(\lim_{w\to5^-}K(w)=\$6.95\), while \(\lim_{w\to5^+}K(w)=\$10.45\).
4. Since the one-sided limits are different, the two-sided limit does not exist. The function has a jump discontinuity at \(w=5\).
Answer
a) \(1.8\,\text{lb}:\ \$4.95\); \(5.0\,\text{lb}:\ \$6.95\); \(5.1\,\text{lb}:\ \$10.45\)
b) \(K(w)=\begin{cases}\$4.95&0<w\le2\\\$6.95&2<w\le5\\\$10.45&5<w\le10\\\$18.45&10<w\le31.5\end{cases}\)
c) Not continuous, because \(\lim_{w\to5^-}K(w)=\$6.95\ne\$10.45=\lim_{w\to5^+}K(w)\).