The diagram shows an open container constructed out of 200 cm² of cardboard. The two vertical and pieces are isosceles triangle of length y cm and width 5x cm, as shown. The open top is a horizontal rectangle.
(i) Show that the volume, V cm³, of the container is given by
Given that x can vary,
(ii) find the value of x for which V has a stationary value,
(iii) determine whether it is a maximum or a minimum stationary value.
I don't know how to solve the first question(i) ?