Analyse and optimise a quadratic model
For a rectangular exercise enclosure, let \(d\) be one side length and let the model output be the area
\[y=-(d-9)^2 + 81\]
The other side length is \(18-d\) metres
The meaningful domain is \(5\le d\le 14\), with lengths measured in metres and area measured in square metres
The output level \(56\) square metres is used for the threshold question