6 The diagram shows a block of wood in the shape of a prism with triangular cross-section. The end faces are right-angled triangles with sides of lengths \(3 x \mathrm {~cm}\), \(4 x \mathrm {~cm}\) and \(5 x \mathrm {~cm}\), and the length of the prism is \(y \mathrm {~cm}\), as shown in the diagram.
\includegraphics[max width=\textwidth, alt={}, center]{66813123-3876-4484-aad1-4bfc09bb1508-7_394_825_459_548}
The total surface area of the five faces is \(144 \mathrm {~cm} ^ { 2 }\).
- Show that \(x y + x ^ { 2 } = 12\).
- Hence show that the volume of the block, \(V \mathrm {~cm} ^ { 3 }\), is given by
$$V = 72 x - 6 x ^ { 3 }$$
- Find \(\frac { \mathrm { d } V } { \mathrm {~d} x }\).
- Show that \(V\) has a stationary value when \(x = 2\).
- Find \(\frac { \mathrm { d } ^ { 2 } V } { \mathrm {~d} x ^ { 2 } }\) and hence determine whether \(V\) has a maximum value or a minimum value when \(x = 2\).
(2 marks)