# Thread: Please help

1. ## Please help

Please help
A square based rectangular prism is to have surface area of 250cm^2 and volume Vcm^3.
I) show V =( 125x-x^3)/2

I get this part but I don’t get part II

II) find the dimensions for maximum volume.

I got the derivative and let it equal zero and got x = root 125/3. Then I got V’’ and found that it is less than zero and therefore a max but how do I get the dimensions. I know the length and width is just root 125/3 but what is the height  Reply With Quote

2. ## Re: Please help

The hieght is root 125/3 too, try subbing it in, both equations they seem right  Reply With Quote

3. ## Re: Please help

Which formula h=125x^2/2x???  Reply With Quote

4. ## Re: Please help THis is from the previous post, If u sub in the values you should get the same answer
EG) Let A = sqrt125/3 (just repetitive introducing pronumeral as a value holder)

A^3=V(formula) = (125A-A^3)/2
Where A is x

Also try seeing the y = (125-x^2)/(2x), if u sub in x = A, u will get y=x  Reply With Quote

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