The constraint is built into the geometry (the original sheet size). We are already down to one variable.
Whether you are trying to minimize cost, maximize area, or find the shortest distance, the process follows a strict, repeatable recipe.
Take the derivative, set it to zero, and solve.
Optimization involves identifying a for the quantity you want to maximize (e.g., profit, volume, area) or minimize (e.g., cost, time, surface area). If this equation has more than one variable, you use a Secondary Equation (constraint) to substitute and reduce it to a single variable. Step-by-Step Procedure