The Application of Differentiation is with the available resource how to get maximum output.
Imagine you have been given with a fencing of length 300 Meters and you are supposed to construct a corral with maximum area with the available length of fencing. 

A=Area

L= Length

B= Breadth

*=Multiplication Sign

A= L  * B

A= (2X)*(Y)

You can divide the fencing into seven section

300=X+X+X+X+Y+Y+Y

300=4X +3Y

3Y = 300-4X