Answer:
,
Step-by-step explanation:
The equations of volume and surface area are presented below:
The length of the tank is:
The expression fo the surface area is therefore simplified into an univariable form:
The first and second derivatives of the expression are, respectively:
The first derivative is equalized to zero and length of the square side is now found:
Now, the second derivative offers a criteria to determine if solution leads to an absolute minimum:
(Absolute minimum)
The depth of the tank is: