V=x(8-2x)(10-2x) or expanded
V=x(4x^2-36x+80)
V=4x^3-36x^2+80x
...
If you were to graph the above equation for V(x) you will see a maximum point where the "y" value is maximized, but be careful as V will increase without bound at values of x that are not part of the domain, ie, in this case x<8/2, x<4 to have any meaning...The proper domain of this function is:
x=(0,4), Mathematically finding this point quickly is by differentiating it with respect to x...
dV/dx=12x^2-72x+80, The maximum volume will occur when dV/dx=0 and the x value is within the correct domain...
12x^2-72x+80=0 (using the quadratic formula for ease)
x=(72±√1344)/24, and since x<4
x≈1.472in (to the nearest one thousandth of an inch)
