A rectangular box has a square base and a volume of 80 cubic inches. If the side of the base has length x and the height of the box is h, express the surface area of the box as a function of x.
Since the box has a square base, so, length = x and width = x, height = h.
$\displaystyle
volume = x.x.h = 80
$
$\displaystyle
x^2h = 80
$
$\displaystyle
h = \frac{80}{x^2}
$
Now,
$\displaystyle surface \;\;area = 2(x.x + x.h + x.h)$
$\displaystyle = 2(x^2 + 2xh)$
$\displaystyle = 2x^2+4xh$
$\displaystyle = 2x^2+4x\left(\frac{80}{x^2}\right)$
$\displaystyle = 2x^2+\frac{320}{x}$
Did you get it now ???