# Math Modeling : One-Variable & Multivariable Optimization

## Example Questions

### Example Question #1 : One Variable & Multivariable Optimization

Using Newton's Method as the computational method, optimize the follow problem.

A cow that weighs 175 pounds gains 3 pounds per day, and it costs 30 cents per day to keep that cow. The market price for cows is 75 cents per pound, but the price is falling one cent per day. When should the cow be sold to make the maximum profit?

It is recommended to wait 7 or 8 days to sell the cow. This should result in a net profit of $131.90. It is recommended to wait 8 or 9 days to sell the cow. This should result in a net profit of$132.90.

It is recommended to wait 6 or 7 days to sell the cow. This should result in a net profit of $130. It is recommended to wait 9 or 10 days to sell the cow. This should result in a net profit of$132.

It is recommended to wait 2 or 5 days to sell the cow. This should result in a net profit of $131.90. Correct answer: It is recommended to wait 7 or 8 days to sell the cow. This should result in a net profit of$131.90.

Explanation:

Assume that the growth rate of the cow is proportional to its weight.

In mathematical terms:

Looking at the known information,

Variables:

Therefore,

when

Thus the differential equation to solve for  is as follows.

Solving by separation of variables results in the following equation.

From here, formulate the model with the new weight equation.

Recall that the goal is to optimize the question over

.

Use a graphing calculator or computer technology to graph the function.

Observing the graph it appears the maximum profit for the cow occurs around  and .

From here, answer the question. Taking into account the growth rate of the cow is still increasing, it is recommended to wait 7 or 8 days to sell the cow. This should result in a net profit of \$131.90.